I think I've already talked about the escape from the knee on belly position that Saulo Ribeiro has taught. I think the key to that escape--and Saulo sort of suggests it--is the location of the knee on belly. Is it a shallow knee on belly, with only a small part of the knee and shin on you? Or is it a deep knee on belly, with the guy putting much of his lower leg on you, as well?
In the first case, Saulo Ribeiro's escape works well. You roll to your side away from the knee, then hook your elbow under the knee and turn quickly into the guy, dropping your elbow to the mat. It works because since most of the guy's weight isn't on top of you, it is easier to move out from under the knee on belly.
In the second case, it might be too hard to elbow escape and hip out from under the guy. With this situation, I like one of the "guard replacement" strategies that Kid Peligro talks about in his book, The Essential Guard.
Here, the goal is to put the guy in your half guard. To do this, you elbow escape away from the knee with your hands on the knee. But rather than try and move your body entirely away from the guy, you just want to move yourself far enough so that the knee that was on your belly is now by your upper thigh or groin area. From here, hook that inside leg around his knee on belly leg. He is now back in your half guard.
Peligro takes this another step further, taking the guy from half guard to butterfly guard. He does this by rotating his hips (opening them up) to make it easy to squeeze his outside leg in between his leg and the other guy's once-a-upon-a-time knee on belly leg. Then slide both legs over and apply the butterfly guard hooks.
Shaka caught me in a knee on belly mount a couple of times Wednesday night. I need to start trying both of these escape variations as the situation warrants.
