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I see what you mean. I agree, "once they get under your chin, it's hard to slip out" - I also agree that "he could just pull his head out" - but in both cases, only if the head/neck grip is the primary grip, and there's no leg/body control.

In my experience, leg and body control make or break a RNC, not head/neck grip: https://en.wikipedia.org/wiki/Rear_naked_choke#.22Body_Lock_... . It's way easier to escape a RNC that neglects leg/body control whether or not they've passed the chin: https://www.youtube.com/watch?v=JR2e0IsqhrQ , and the chin is an important part of the last-ditch defense.

...which is a long way of saying, I'd rather fight a dude with no chin, and if there were some option to temporarily remove my chin while grappling, I'd still decline, for performance reasons :)

I'd love it if a more experienced MMA player could chime in to confirm or deny our suspicions.