He simply pushes some one standing completely off balance. It doesn't require much force, just the amount generated from placing your hand on some one and twisting you're shoulder.

In my boxing gym my coach will get lads to step in teh directions he says until he tells them to stop, no one is aloud to move they're feet and he'll take a look at you're stance and push you if you're off balance.... Obviously they don't fall down because they're in some form of stance but it makes them stumble (usually back into stance as they over compensate trying to not fall over)

Because you know the chair is there when Bruce lee does it you're not as worried about falling so sub consiuosly you make less of an effort to stand up.

http://www.metacafe.com/watch/23745/bruce_lee_training/