Just a guess. Could it be that the striking leg, being free to vibrate, keeps more of the energy that it's trying to apply to the anchored leg. Basically, I'm saying the kicking leg does a whipping action around the opponents leg and bends, with the foot acting as a weight on the end. The power really is in the swinging foot but the contact is higher up in a weaker spot, besides being illegal. If you get a stick with a metal ball on the end and whack a pole, with the ball making contact with the pole, the pole will break but get closer, such that the stick makes contact instead, the stick will break because the stick is not strong enough to stop the ball which wants to carry on.