"Black Bear Sinks Hips" is a good way to fell an opponent at his back

A big mistake many martial artists make when their opponent kicks, is to move back. This hands the initiative to their opponent. You need not move back. Just slant your body backward, without moving your feet, to avoid the kick. However, if the kick is close, you may move your front leg backward without moving your hack leg. As the kick has been spent, preferably before your opponent has recovered his balance, swiftly move in to fell him, and finish him with a decisive strike while controlling him on the ground.