A waterfall starts with the plane's wings level with the horizon and completely stalled. He entered it not stalled at all. It's not a waterfall. It looks more like a forward moving knife edge spin.

Whatever it is it's very cool. I noticed it right away when I saw the video. That guy is a great pliot and of course flying a great plane, CAP232 (a true tumbler).

I saw the Oracle Pitts and Bull Dog do a really cool one that's not easy. It's like a harrier but with the plane yawed almost 45 degrees. They do it right off the ground (8 feet) and it looks cool as hell. Try to do that one!!

The only thing I can think of how they do it is rudder for the yaw and opposite aileron to keep it flat (almost full up elevator of course). It looks like a sideways harrier. I wish I could find a video of it.