Are you talking about this?
http://www.youtube.com/watch?v=yxxb2ctulEs#t=2m
I have seen taijiquan practitioners also do the "push-on-head" demonstration. I have always thought it was an example of grounding - realigning the bodily structure so that any incoming force is redirected through the body into the ground. I'm definitely the least qualified here to discuss it; Mike Sigman or Dan Harden or a bunch of others definitely could explain it better.
Ultimately, those four guys weren't pushing on O'Sensei, they were pushing on the ground. In fact, it looks like by their postures that they were relying upon O'Sensei to support their weight. He didn't really throw them in this video; he moved out of the way and they fell because they had nothing supporting them.
Could anyone do that? Probably very few can. I know I certainly can't. But from doing yoga headstands, I know the body, if aligned properly, can transmit incredible forces through it without really being affected by them.
I'm content to think of O'Sensei as an extraordinary man who's
mastery of these body skills was bordering on the supernatural, but I also think that the
skills themselves were not magical. The way you can say a genius concert pianist who just improvised a moving piece seemingly out of thin air is divinely inspired, but the playing of piano itself is just another amazing thing that humans do.