The final two punches in Jion are described as straight punches out to the side. With very little exception, however, videos show people doing a cross between a straight punch and a hammer. Some bunkai interpretations explicitly turn this into a hammer fist. Since everyone is doing it, I'm wondering if it might make sense to give up on the straight punch interpretation and just think hammer fist at the end of the kata? Any pro's/cons in joining the hordes, abandoning the straight punch explanation, and doing the hammer fist that everyone else does, as opposed to what they say?
That weird looking hammer-fist / straight punch combination is done for a reason and should remain that way in order for the bunkai to work. I'll explain below.
Just to preface this, so you know where I'm coming from, I discuss the purpose of karate kata in depth in my answer in the following link, which I recommend reading:
I also wrote a bunkai for a movement in Heian Shodan at the following link, which you might want to read first in order to get a feel for kata bunkai:
Now, as for this technique you've spotlighted in the kata Jion, it seems to me this section of the kata is about take-downs using an opponent's extended arm as leverage.
The movements preceding this show stomping out and hammer-fisting downwards. I'll dispense with a huge description like I normally do, and I'll just say what's happening there is pretty obvious if you've been exposed to any form of grappling. This is an arm bar and simultaneous elbow break. The stomp that you do with it is to sweep / kick his leg. If you do this all at the same time, you're pulling his arm towards you (that's what your chambered hand is doing), you're unbalancing him so that you can easily bend him over, and you're breaking his elbow (or at least putting pressure on it). You're also sending him face first into the floor at the end of this.
So the "feel" of that bunkai is that you're using his arm, extending it towards you while breaking his balance. From there, you use it like a lever to bring him down to the ground, face first. The elbow break you could get is optional, and pretty easy to do with that hammer fist strike.
The next movements after that are the techniques you're interested in. Here, you begin by opening up your hand, sweeping it up and then down. Then it turns into a fist as you pull it from a forward position to a chambered position ending with your fist close to your nipple and with its palm facing down. Your feet are close together here.
Then as you're changing from an open hand to a fist with that one hand, you're extending your other arm outward to your side. When you do this, it seems weird, because it's a combination of a straight punch and a hammer fist strike. And you're stepping out with one leg, often shown with a slight hop or slide.
What's actually going on?
Remember when I said this section of Jion is concerned with take-downs utilizing your opponent's extended arm? That's what's happening here. Except, instead of sending him face first to the floor, you're now sending him backwards to the floor. In both cases, you're using his extended arm as leverage. In both cases, there is an optional elbow break that's possible.
The application I have in mind is similar to the one shown in the following video at 1:40 (his fifth elbow break), except he doesn't show the throw:
Notice on the video that he's holding the guy's wrist up high on his chest while his other hand snakes outward in front of the opponent's chest. This puts pressure on his elbow and locks him at his shoulders as well. Notice where his legs are in relation to the legs of his opponent. The throw I'll eventually describe happens easily from here, if you just push him backwards, which is what the hammer-fist component of this technique does in the kata Jion. Your legs will be right behind him, and he trips right over them. I'll explain this in detail now.
Let me give you a self-defense scenario to go along with it. Suppose someone is standing face to face with you. He grabs your right lapel with his left hand. As he does that, he twists your lapel. His left elbow is now bent and pointed straight down. His left fist has its palm facing upwards. He's pulling you closer and is about to punch you in your face with his right hand.
This scenario comes up quite often in typical bar room scenarios, muggings, and whenever things get out of control between two people arguing. It would have been seen back in ancient times just the same as it is today. Nothing has changed.
That's what this last part of the kata is for. You first turn your body sideways to him, pivoting 90 degrees counter-clockwise on your right foot. That takes your left side out of the direct line of fire from his right hand.
Next, you lift your left hand up, coming over the top of his left hand, grabbing his left hand's wrist from the inside on top of it. Your left hand will grab his left wrist at the underside of it, but your left hand is above his wrist. Then you pull that wrist straight across your chest, slightly twisting it outward to release his hold on your lapel (it's not imperative that he releases, however). That looks exactly like what's happening in the kata.
At the same time, you're snaking your right arm upwards and outwards, looking like a weird combination of a punch and a hammer fist strike. And you're taking a step out (or a slight hop) to your right side while you do this.
What you're actually doing is extending his arm out, elbow facing down, while gluing it to your chest. It locks his elbow and his shoulders. You accomplish this by pushing forward into him with your right hand while pulling his arm to your left with your left hand. You're kind of stretching his arm out. This has the side-effect of twisting his body 90 degrees to face the same direction as you.
That's what you're doing with this straight-punch / hammer-fist combination technique. You're snaking your hand up and through the hole between your opponent's bent elbow and his torso. That's why you can't just hammer fist here. If you try to use a pure hammer fist, it will not be able to connect, because his bent elbow prevents you from getting through to his torso! So the pure hammer-fist strike can not work in this scenario.
While you're extending out his arm, you're stepping slightly behind him at the exact same time. Now you're in a good position to push him backwards over your legs, sending him to the ground behind him. That's what you're doing with the hammer-fist component of this technique. It's pushing him backwards over your feet.
Remember that you extended his elbow into a locked position doing this. By shooting your right hand upwards and outwards at the same time, while pivoting your body 90 degrees, while pulling his arm with your left hand, you can get an elbow break here if you want. Do it quickly and with power, and it can snap that elbow. Do it more gently, and it will merely lock the elbow and shoulder.
The reason why you're pulling his hand to your nipple and not downwards to your waist like we see so often in karate kata is because you can't. His grip on your lapel is strong, and you would have no leverage to be able to pull it to your waist. It wouldn't work. But you can pull it straight across your chest, just like we see in the kata.
By locking his arm out in this position and turning his body to a side-by-side position, you prevent him from being able to move. You're in control of his body movement now. You've locked both his elbow and shoulders at the same time. He can not avoid what you're about to do next: You push him backwards over your legs which are behind him.
Notice also that, because you've forced his body to turn 90 degrees, he's already moving in the direction you want him to go. He's already going to trip over your legs. There's little effort needed to push him over.
So the hammer-fist / straight-punch combination technique has a very specific purpose here. It won't work if you turn it into a pure hammer-fist strike, because you haven't extended his arm in order to get his elbow out of the way. You haven't turned his body to a side-by-side position, either.
What about making this a pure straight punch, instead? In that case, you would have to target his face, because it's the only thing open on your right side. But that does nothing about his lapel hold, and it does nothing to protect you from his punch he's about to do. It's not very smart.
This technique would work pretty much the same way if your opponent was behind you, also. Suppose he was behind and to the right of you, grabbing you with his left hand. He's about to tug at your shoulder so you'll turn to face him. Once you turn to face him, he'll punch you in your face with his right hand. This is known as a "sucker punch", and it would have been as common back in ancient times as it is today.
In that scenario, you reach over his left hand to grab him at his left wrist from the upper side of his wrist instead of the lower side like before. Then you pull it across your chest as you release his hold with a slight twist outward. You elongate his arm this way and by stepping behind and towards him while extending your right arm out in the straight-punch / hammer-fist combination technique. Now his elbow is once again in a position to be broken as you send him toppling over your legs backwards to the ground.
Once again, if you tried to pull his left hand down to your waist instead of your nipple, it would have caused him to bend forward. That would cause him to step forward and would allow him to bend his elbow and turn towards you, avoiding the take-down and getting into position to grab or strike you again. So that won't work. You definitely need to chamber his hand high up on your body.
So we saw in Jion that there were two ways to utilize leverage by extending your opponent's arm from a side-by-side position. The first is to send him forward, getting an optional elbow break at the same time. The second is to send him backward with an optional elbow break. Both techniques also employ a sweep or trip to help unbalance him.
Now consider the possibility that you first tried one of them, and it didn't work. Then you can immediately adapt to it by changing to the other technique. So if you're attempting to send him to the floor face first, but he straightens up and avoids the take-down, you can immediately switch to the technique that pushes him backward over your leg. Or, if you're attempting to send him backwards to the floor, but he manages to avoid it, you still have that arm, and you can sweep his leg out and hammer-fist strike downwards on his arm to get him to go to the ground face-first instead.
Now, what if he grabbed you with his other hand instead (the "wrong" hand)? Well, give it a try. It changes the technique, but that's for you to figure out, because it's not shown in the kata. And keep in mind, if you end up in trouble by trying it on the wrong hand, you can switch to the other technique to save you, though it will cost you some valuable time. You should play around with making this "mistake" and figure out the right thing to do.
Both of these movements shown at the end of Jion can be used with either attacker's hands for different effect, and either movement can be used to back the other movement up in case something goes wrong. If you play around with all these different scenarios with a partner, you can drill this into muscle memory so that you don't have to think about it in a real fight. Make it a flow drill if you want.
As I mentioned, hidden (or not-so hidden) in each of these two movements are elbow breaks. Elbow breaking is one of karate's hallmarks. It's done throughout karate kata, but often as an optional technique. It's not required, but it's there if you need it. Why would you need it? Maybe if your opponent had a knife in his hand. The elbow break is shocking and will probably cause the attacker to release his knife automatically. If you don't break that elbow, then you have to disarm that knife after you throw him to the ground, which isn't karate's forte.
This is why you really need to be careful when practicing this on a partner. Go slowly and gently. Torquing the elbow and rotator cuffs are so easy to do with the leverage these techniques apply. Adding speed increases the likelihood of a serious injury by accident.
That's what I saw. I could be wrong.
One final thought...
Not knowing the bunkai for kata means that errors will be introduced into the form over time as people try to guess at what the movements are supposed to be doing. Notice how in this case, the original poster was not sure if it was a straight punch or a hammer-fist strike. It's neither, and so it seemed sort of weak and imperfect to him. He was going to go ahead and fix it by substituting it with a pure hammer-fist strike instead. That's what he saw others doing in their interpretations of it, so naturally why not make it official and change the kata itself?
This is why variations in kata exist today. When you don't know the original bunkai, then the form becomes an inkblot test. You will see whatever is familiar to you. And that will lead to mangling the form over time.
The truly frustrating thing about this is that there is no "official" bunkai for karate kata at all. That's because no karate organization has attempted it, at least not to my knowledge. And perhaps that's because nobody can really say what the original bunkai was for each kata. It wasn't written down. Typically it was only taught to high level, trusted students, if at all. With each new generation, there would be less and less people who actually knew the bunkai. So transmission is nearly completely broken at this point. I don't think anyone alive today can definitively trace their bunkai back to the founders of their kata.
And this is not just karate. Many if not most kung-fu styles have the same problem. FYI.
Form dictates function, and vice-versa. Looking at this technique from the kata Jion, it seems pretty likely that my bunkai was the original one. Why? Because if you study classical jujitsu or any grappling art, you will recognize the motion. There aren't many possible things it could be doing, given the mechanics of the technique. So it's likely this was the original bunkai, or at least the main part of the technique (the creator of the kata probably had his own variations and subtle details that made it more effective).
For other movements in kata, it's not so obvious. You see a lot of down-blocks in karate kata, and those can have dozens of interpretations. You have to look at the movements immediately before and after them to get a feel for what might be going on. But even then, it's not so clear.
Hope that helps.