Thumbs up, so palm facing out? My initial thought is that the guy on top, or bottom guard even, can push outwards against the hand, wrist locking your arm free. palms in the wrist lock is blocked by the other hand, if you are turning your palm outwards it should be easy to apply pressure against the wrist by grabbing the fat of the hand/thumb and forcing the hand out against the arm. I'll look at it though later. Pics/video of what you are doing would help.
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