that technique is taught at my school, and it's great. It's basically just paying attention to a guy's weight. While someone is dragging the knee into mount, they have to take at least a little weight off of the other knee for a quick moment. If you elbow escape to that other side, even if the guy on top is heavy, he won't be weighting the leg and you get an easy half-guard. Also, if the guy has the mounting knee high, you can dive right into deep half with that.