This wierd hoppy feet-lift is something I've noticed over the years on Taiji-related videos, it's a movement not normally necessary for regaining balance after being shoved backwards ... I just concluded it was a learned response which is encouraged by their authority figures and peers. Is that the whole story?
I would tend to believe that except that I once did it to a guy who had no tai chi training at all. He was there to enquire about my aikido classes and I started showing him some sticky hand (chi sau) things when he decided to push me with both hands. I circled under his hands and pushed back very softly, but something about the interaction caused him to lift off both feet and pop backward several feet, about 30 inches off the ground, to land on both feet about five feet back of where he started. I wish I could do that at will, but I can't. Which is why I've been trying to learn about IP/IS or whatever else it may be called.
On the other hand, a lot of what you see is just crazy. Like most aikido demos you see where people fall for no real reason and the sensei gives that smug smile as if he actually caused them to fall.
I did have a student grab me once with a reverse single-side grip. We were only supposed to be doing hand-escapes--getting out of his grip. But when I tried to escape, he just had a really strong grip. So I just "surged" through the movement and he came off his feet and did the feet-over-head flip in the air to land on his back.
The way I know this was real was that it was a concrete floor and we had no mats at the time.
I've had other bizarre little things like that happen over the past decades, but, sadly, I've never learned to do it by will.
On the other hand, some teachers train their students to be sure that it always looks like they're that effective. So, yes, some of it is involuntary and uncontrollable, but probably the vast majority of it is fake. As for the guy in El Paso, if I ever get out there, I'll let him do it to me and form my opinion from that.
I hate fake falls and screwy jumping responses and I've assiduously trained everyone I've ever worked with NOT to take baloney falls if a technique is no good. But every now and then something happens so easily and produces such a startling result that I know there's more to this stuff than you might really understand just by seeing it happen.