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):
- Tuck the chin into the crook of the elbow. This is all or nothing, so force your chin into that space.
- Slip your hands inside their thigh. This will create a minuscule amount of space.
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:
- slide down.
- open his legs due to the strain.
While you're creating space, flex your elbows out to create more.
- Shoot your legs to one side. Preferably, you're going to shoot in a direction opposite where your chin is pointing. If he has you in RNC with his right arm, your chin will point slightly right, so shoot left.
- Bring your hands back up to pull on that elbow.
- Pull him down. Sliding down to the wrist/forearm as you pull that elbow down to release your head will give you control through mechanical and minimal pain compliance.
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.