Oh gosh, the answer is so complicated that I literally ended up writing far too much and then deleting it and started over again, because I know nobody will care to read that much.
So I'm going to cut to the chase and hope that it will suffice. This will probably still end up being long. But so be it.
Basically, the rising block that you see in tangsoodo, karate, and taekwondo is not a block.
Form dictates function. And I think the reason why your instructor has modified the rising block is because he figured out that the regular rising block doesn't work as a block. He believes the function is either to block a straight punch to the face or an overhead strike downwards with a knife. He quite correctly and reasonably deduces that the form is totally wrong for that function, so he modified the form to fit the function. Very smart guy, your instructor. Kudos!
But there's only one problem. The function your instructor has in mind is wrong. It's not a block to a straight punch to the face. It's not a block to an overhead, downward strike with a knife. Sorry, those are not what it's for, no matter how many times he's heard it from his master, and from his master's master, and so on. Yes, they're all wrong. Isn't that a hoot?
How do you know that's not its function? And if it's not a block to a straight punch to the face, what is it?
First off, have you ever once used a rising block in sparring? If you did, how did it work out? And why haven't you used it since then?
The answer is, nobody uses the rising block to block a punch to the face. It won't work in sparring, and it won't work in a self-defense scenario in real life either. At least, not as it is taught.
About the only time I see the "rising block" used in sparring is when someone wants to lift up and duck under someone else's arm, and then do a quick reverse punch to the abdomen for a point.
Yeah, that kind of works. But note that whenever you see that, the rising block isn't done the way it was taught. First, instead of doing a rising block to an opponent face-on, you actually have to step in from a diagonal angle so that you avoid getting punched straight on in the first place. Second, instead of the rising block being done to a straight punch, you're actually doing the rising block to a dead arm instead. In other words, you're just lifting up the other guy's arm and ducking in underneath it so that you open up a nice hole in his defense for you to sneak in a punch to the abdomen. Then you keep your arm up in that rising block position as a kind of shield. Nice going! But also totally not a rising block!
And what about the rising block against someone who comes swinging straight-armed with a knife downwards to your head? The answer is: That is never anything anyone armed with a knife ever did. Nor anyone who just wants to hammer-fist strike you. The idea that someone will attack you in that way is just laughable. It never happens. That's not what the rising block is for.
So okay, what is the real purpose of the rising block?
The answer to that question is somewhat deep. The rising block comes from forms, not from sparring. That is its origin. So the answer involves understanding that forms actually contain self-defense techniques, not blocks.
These self-defense techniques come from Okinawan shorin-ryu karate, because that's where Tangsoodo's forms came from. And Okinawan karate encodes self-defense techniques in a series of 1 to 3 movements in each form, in general.
To understand what the rising block is doing, you have to realize that it can be used in many different ways, depending on where it is being used in each form that uses it. So it requires a good understanding of something called "kata bunkai" (in Okinawan karate terminology). Kata bunkai just means "form analysis". It's a way of breaking down the movements of your form, analyzing them, trying to determine what they are actually doing.
I'm not actually going to give you the entire lecture on how to interpret forms for self-defense, and how to do kata bunkai. If you google "kata bunkai", you'll probably pick up a ton of information. That's fine.
Just know that what the forms have in them are encoded. They're hard to figure out. But once you know what to look for, it's like everything makes sense. You'll see it. And you won't have to modify anything about the way you're doing things in the form in order to make it work. The rising block's form will not need to be changed at all.
Like I said, there are no blocks in the forms. You learn blocking by practicing sparring with a live opponent. You don't learn blocks by doing forms. Instead, the forms are about self-defense.
Self-defense is a vague phrase, though. What do I mean by self-defense? I mean that these are situations that you will find in every culture and every time period, common scenarios that everyone finds themselves in.
Examples: 1) Someone comes up to you and starts screaming in your face. He then grabs your lapel with his left hand and cocks his right fist back ready to punch you. 2) Someone puts you in a bear hug from behind. 3) Someone grabs you on your shoulder from behind, tugs at you, and starts to punch you. 4) Facing you, someone grabs your wrist with one hand and your throat with his other hand. 5) Someone puts you in a side choke. 6) Someone comes up behind you and grabs your left wrist while either choking you or sticking a knife against the front of your throat with his other hand.
These are typical self-defense scenarios. They are universal and common. And the forms are designed with them in mind. When you see a rising block in the form, it's not blocking a punch to the face. It's doing something else.
In Tang Soo Do's first Pyong Ahn form, you'll see rising blocks. Notice how they are being set up. What are the movements immediately before the rising block? There's a down block. Then a knife-hand block. Then you do the rising block. What does it mean?
Here's one possible interpretation for those movements in the form. After the down block is when the self-defense application begins. In that position, you are being grabbed on the wrist. Imagine someone grabs your left wrist with their right hand. You do an inside to outside knife-hand block with your left hand (just like in the form). What does that do? It is telling you to circle your hand up and outwards so that you break his hold on your wrist. This is called a "dissolve". Look it up.
But you don't stop there. With that same left hand of yours, you grab his right wrist. The palm of your left hand is now against the underside of his right wrist. Now you do the "rising block". Here you maintain your hold on his right wrist with your left hand. The form tells you to cross your right arm in front of your left arm from underneath, and then you rise up to do the rising block. At the same time in the form, you are pulling your left fist back, chambering it at your left hip. What you're doing is pulling his arm towards you, twisting it and extending it so that his right elbow is pointing downwards. Meanwhile, your "rising block" is putting pressure against that elbow of his. As you step forward, you make a sudden jerk with both arms, and it breaks his elbow! You can simply let go of his arm if you want, and instead of breaking his elbow, you will merely get his arm out of your way. Or you can just stop at the dissolve if you want. It's your choice.
That is the "rising block". Notice, no blocking. Just bone breaking, grappling, and positioning. Good stuff!
Anyway, hope that has given you something to think about. When I first figured this out, it was like stepping into a whole new world. Keep in mind that you will either need someone to show you everything and trust them, or you need to find a good classical jujitsu instructor to teach you this stuff so that you can see it in your forms. Because, that's what it is.