I really like this, and I am going to play with it. I like transitional positions, which is what knee-on-belly is anyway. When you do the standard basic mount by driving your knee across your opponent's belly, the weak spot is usually the other knee, because you take your weight off of it and the guy can elbow escape right back into half guard. The grapevine seems to tighten that up and let you stay for a second. I'm pretty sure this has at least some merit - the guy is a professor under Gracie Barra, and even though they are my gym's rivals, they are extremely legit.