A while back, my wife told me about this dance move that famed singer Freddie Mercury did in some music video or concert footage. I don’t know. I can’t dance and I don’t really watch music videos, so I’ll just have to trust her on this one. Anyway, the move he did was apparently very similar to the way I get the underhook here. The big problem I have found is when people try to get an underhook from bottom, if you lead with your fingers then the elbow tends to flare out which allows your opponent to easily re-pummel for their own underhook or at least jam your hand up with top pressure and cramp your arm position, which sucks to be stuck under and shuts this whole game down.
The key here is to keep your elbow pinned to your rib and swivel your hand around that fixed point, like the hand of a clock spinning around the center point. It’s also important to note that this is really hard to do if you get flattened out on your back, so whatever you do, make sure you aggressively get to your side first!