This is one of the best techniques in Sombo I have ever seen (and used)! When you are in the half mount, place your inside leg against his outsde leg and tuck your calf tightly behind his knee. Begin to hook your ankle behind your free knee.

Shoot your arm through his outside arm and bypass his outside arm (similar to a standing wrestling technique). Grab his belt or pants or hip and pull yourself as you sit up quickly (and put pressure on him trying to hold him down.

Grab his foot and pull back HARD! Make sure your legs are all tight together and your ankle is well locked behind your knee.