Within the context of Brazilian Ju-Jitsu and its rules:
Assuming you are on the ground and someone has your back and hooks in, is there a defense to a rear naked choke? Or is the only defense to stop the choke before it can be put on? or is there good defenses when its only partially on? or ideally is there a tactic to use when it's fully on?
Depending on if the choke is on and how their arms are positioned here are a few options I'm aware of and try to work on (these are very brief descriptions, barely scratching the surface of back and RNC defense, to get you started on further research):
Hooks in no upper body control - Protect your neck with the "V" "prayer" position , flare your legs out and keep scooting down, once you feel you are low enough so they can't pull you back up straighten one leg and kick off the hook and spin to your knees away form the side you just kicked their hook off. This is a short summary of the Saulo Ribeiro type escape on his dvds and book.
Hooks in with "seatbelt" upper body control 1 - Pull their top choking arm down to get some space, turn and tuck your chin into their elbow. Bridge and roll your body in the opposite direction of their choking hand, your goal is to get your head and back to the ground then shrimp and regain guard or half guard.
Hooks in with "seatbelt" upper body control 2 - Get a "wedge" or "frame" under their clasped hands, using your own arms and frame their grip off, get control of their top choking arm get it to the other side of your head then spin around to your knees to be in their guard.
Hooks in with RNC sunk in - Tap ;)
Assuming you are on the ground and someone has your back and hooks in, is there a defense to a rear naked choke?
Not everyone is going to agree with me here, I can guarantee that. I am not familiar with competition rules for BJJ, but I understand them to be fairly liberal, with the exception of small joint locks and soft-tissue mauling.
With hooks in, you're pretty well S.O.L. If he's got you from behind, hooks in, and is already applying an RNC, you've screwed up badly. Here's what I'd do (assuming I were competitive; I'm not, and would inflict a great deal more pain on the way):
Arch your back. Remember that small amount of space you created before? You're creating more space by keeping your hands in their space-making position, and opening your body into a spot where he has to:
While you're creating space, flex your elbows out to create more.
Remember, at this point, you're losing. You're going for a hail mary, so really commit. This will only work in rare occasions where you can flow from one point to the next and fully commit. Further, if they're aware of what you're doing, you're hosed.
NB: This is a ground variation of a standing technique used in the Bujinkan, and may need to be adjusted a bit to fit the BJJ rule set.
Or is the only defense to stop the choke before it can be put on?
The best defense is being able to defend against it before it happens. This is about awareness and applying a quality defense. Unfortunately, you have a lot of restrictions working against you because of the rules.
If you can protect your carotid artery, you're going to remove the effect of the RNC, which buys you time. The secondary danger (and an extremely common one) is the neck crank that can accompany it. Practice everything in a controlled manner before you apply it. If you're not flexible enough, keep training until you are. The neck is a really dangerous playground.
or is there good defenses when its only partially on?
Definitely. The less on, the more options. As you see it coming around, you have the option to adjust into an arm bar. Further on, you can get most of you face down into the elbow and you're half out already. From there, you can control the ankle.
There are tons of options. Explore them at various points in the technique by training slowly with a partner to discover your options. Get into one position, begin the technique, and say, "Stop!" at different points. Explore where you can go and how the body moves when restricted at various points. Part of what makes good martial artists into great martial artists is their ability to explore their body's capabilities in various ways.
Assuming it's a traditional RNC, locked in.
Look at his wrist (obviously not the one that's behind your head), this will bring your neck to his bicep, and you'll have better leverage to pull on his wrist to give yourself a bit more space. Don't look at the elbow and don't try to pull on it, that's an exercise in futility.
Having given yourself a quick breather, you now need to deal with the hand behind your head. Since he's got both his hands occupied you don't need to worry about a wrist lock so you can afford to reach your hands back where you can't see. Grab his wrist and pull it in front of you where you can see it.
Go back to pulling on his arm that's across your throat - wrist again, as far from the elbow as you can.
At the same time as you're using your hands fighting his hands, keep working your back to get it to the mat. Once your shoulders are on the mat, the choke threat is gone (unless you're wearing a gi, in which case there's still collar chokes, but you can defend that by grabbing your collar, which also protects you against another RNC attempt.
If it's not fully sunk in yet, then what you're doing is hand fighting. Every time you see his hand cross your field of vision grab it and pull it away or slip your hand in between your neck and his hand and push it away. Make sure you're always keeping your hands where you can see them - if you reach behind then you're at risk of getting wrist locked.
While you're doing this kick your feet up and forward, dig your heels into the mat and pull your body forwards. You'll be getting your armpits closer to his thighs, making it much harder for him to get the choke in.
At this point, same strategy as for the first one, you want to work your shoulders to the mat. Pick a leg of his, put your body weight on it so it's pinned to the mat, and if you have to bridge a bit to get your shoulders on the mat - do so. Once you're there, the threat is gone. But make sure your weight is still on his leg, otherwise he'll very easily take the mount.
From here, thread the needle to get on top in his guard. Threading the needle involves keeping one foot flat on the mat, and lifting the other foot and pushing it through the point between your calf and hamstring. This will naturally make your hips turn, so turn the rest of your body with it. Make sure your arms are t-rexing so you get your palms on his hips as you come up.
If in a match, all you can do if they have their hooks locked in, is tuck your chin and hand fight, and hope time runs out. AS said above, there are a lot you can do if its a street fight. The best defense is try not to get your self in that position. good luck.
I have seen some people grabbing the little finger of the attacker and pulling it away from the neck. That puts the attacker in immense pain and he is bound to leave the choke.
You pull their wrists down after doing that your chin shoots down to your chest whilst your shoulder shoot up. You can then start to take their hooks out if you are on the ground if standing up hip throw.
Two hands in at all times no matter what and shrimp your way out. Don't push with your hands. The position is similar to the thai clinch. This has saved me from being subbed from black belts so many times!
If you are in the street some options are:
