Only just read that, Mr. Whippy, and decided to give it a try. (In the bathroom - with an ordinary bedroom slipper. There because the room is on my home's ground floor in a sort of "South Wing", so my neighbour won't hear it through the rather thin wall!)

I found it difficult to strike fairly hard and evenly, and started to feel a bit dizzy from the head-down position, but even that very mild slippering left a gentle tingling that lasted about 5 to 10 minutes.

Then another go, first with a beach "slipper" consisting of a thin neoprene upper on a rubber sole, then with a heavier plimsoll. The latter ought to have been effective but I found its size and weight made it awkward to swing properly.

I will have to find my rubber paddle, which I hid somewhere so safe I can't recall where! I made it from a scrap of quarter-inch thick, moderately stiff rubber sheet I found at work, and its blade is about 6" long X 2" wide, with semi-circular end and rounded edges.

So thank you for the idea, which I will try further!