Cross leg kick drill, it is the back leg of the stance and yet it control the center of the gravity of the whole body. It kick to the knee joint and below of opponent's leg.
Besides you have understand how to apply cross leg kick drill, you have to know how to get yourself away when opponent apply the same technique to attack you. It's simple and fast, just turn your front leg backward and controlling by Yew Ma (Waist & Stance) and become center of gravity for the whole body.
