It can work like jakuzi. Water pressure and rotation. Perhaps with suction. Perhaps with slow brushing.

All kinds of machine-assisted things do much more than you can with your own effort, without getting extreme. Constant slow brushing with no danger of ripping anything out, with constant pressure and suction interchanged, for like 20 minutes without stopping (you wouldn't clean yourself well enough for 20 minutes without stopping) and with all the expected gels and such - and you're clean enough without much effort. Would also be economy of water, I'd expect.

Not economy of energy, of course. But energy seems to undergo inflation in our world, while water - the opposite. So.

EDIT: Also when I'm thinking about it, such a machine can even be made work with purely muscular effort. Like a mechanical lawn mower. Still more convenient than having to reach for every place on your back after sitting 10 hours straight behind your desk and with having a migraine.