If you do a Youtube search for Empty Force (or Ling Kong jing), you'll see all kinds of crazy looking stuff where people move without being touched. I think the level of misunderstanding here is a couple of generations deep and I'm not going to dig all that up right now, except to say that what we call 'a magic trick' in the west is sometimes called 'qi' in the east.
A subset of these videos are showing something very real, which is easily misunderstood. I bring this up now because of a conversation I had with Majia about display which she has turned into a very clear blog post. If someone whips out a knife and people jump back, nobody says, "Hey, they just got blasted with qi!" But people generally have an instant and intense sense of the distance they need to be from a razor sharp knife in the hands of a threat. People will jump back in a lively way even when they are just playing with knives (Jumping back, by the way, may put you in a worse position.)
Knowing what is really a threat and what isn't comes from training, practicing and playing with others. Baiting, feinting and small but deadly shifts and changes are invisible to the uninitiated.
The uninitiated can still be scared, but not by the same stuff as the expert. This is much more obvious with a knife than with open hand because most of us are at least a little scared of a knife. It also follows that people who play with knives are likely to be sensitive to the difference between subtle but deadly, and showy but impotent.
But if you have a teacher whose punches hit with the force of a sledgehammer, and from playing and practice with that teacher you are aware of tiny subtle shifts that signal a real attack, you're going to get out of the way fast. An outside observer is unlikely to know why you moved. Then imagine that this powerful teacher uses this ability to get you to move, and then to get you to move again before you've finished moving, and then again! One of the real possibilities is that the trained student will feel their only choice is to jump backwards and roll away. And the illusion of "empty force" was born.
Display is obviously part of the "monkey dance" category of fighting that young men are so prone to be possessed by. But it is also a very real part of self-defense. There is a whole category of street-level predators who use display in various ways to test whether they can get close enough to make you into an easy victim.