This sexual act requires 4 people: at least 1 guy and 1 girl; most preferably 3 guys and 1 girl due to the physical strength required. A Kellner Washing Machine (KWM) is preformed by having one guy standing in a pool with his penis in the girl's pussy and she is curled up in a ball so that the 2 remaining people can spin her around and back and forth, sloshing the water around as in an actual washing machine. The girl may want some good goggles. Depending on the guy and the spin methods employed, the girl may also need a breathing apparatus.
I got the most amazing KWM (Kellner Washing Machine) in my pool last weekend. I didn't know Lauren could hold her breathe that long. It wouldn't have been possible without her friends Christina and Niki.