A fun stroboscopic effect. The water isn’t really spiraling, of course, the effect is because the frame rate and sound wave frequency are the same, so the locations here the sound displaces the water (or doesn’t) is the same whenever a frame is recorded. Or close to the same, when he goes 1 Hz in either direction. Same effect as wheels looking like they are revolving backwards in some movies. You’re sampling images at the difference frequency.