The feedback is the only thing it’s really doing. As in, you’re picking up the sound of whatever it’s on, then pumping back into the same surface.
I don’t see any videos using it on his webpage, but last time I saw a friend of mine (Luigi Marino) perform he had some DIY versions of the same thing on his huge china cymbals. He basically used them as acoustic oscillators as they would end up tuning into nodes of the cymbals, and would change as he moved them around.