Watch a video of a small waterfall. The hydraulic (hole) is at the base, where the water gurgling up from the bottom feeds back into the base of the falls. Real powerful ones can keep big boats, logs etc recirculating for a long time and a swimmer may spend more time underwater getting gurgled than on the surface where they can breathe. Those small boats can get caught and flipped in fairly innocent looking ones like this gif. The kayak punches right through it with speed. It's hard to punch them like this on little pontoon boats. The hole catches the stern end and tends to flip the boat backward.
