i thought i mentioned in passing by way of the runner example, but i guessed i need to spell it out a bit clearer.
if you watch some of the soccer plays, you would see a soccer player running full tilt and if another player body check him/her from the side, the running soccer player would often goes tumbling into the grass. IS folks basic training is to be able to deal with forces coming at many directions at the same time - up, down, front, back, left and right - to name a few. so from my point of view, IS training, at the foundation, is multi-dimensional force neutralization. now, if normal athlete learn such training, would make them much better athlete than before. the soccer player would keep on going and score!
(yup! me likes soccer!)
Sorry Phi, I must be in some kind of mood today, but a body in motion where that body is in the air (such as a running athlete or IS master), at some point, can be toppled as easily as the next guy. If there's no ground to support or act against, then all you're dealing with is momentum that can be be redirected, especially if you're talking about a force directed from the side. Perhaps walking would be a better example. At least one foot is on the ground at all times.