Decades ago, I was introduced to it in a Scientific American article, which used it as a demonstration that it can be necessary to make two loops to make a full cycle. I think the article was actually about something subatomic, and I think it's reasonably likely that someone reading this will remember what the article was actually about. Anyway, the article said it was a folkdance move. Does anyone know which tradition it's from?
IIRC, it's done with a bowl of water in your hand. And I'm not sure if I'm making this up, but I think that if you're really good, you can use a lit oil lamp.
wolfdancer has mentioned the move as a good way of improving flexibility, and I think she said it shows up in belly dancing.
I was just introduced to a more ambitious way of using it--lean as far as possible in the direction your hand is going. This is enough of a weird stretch that a little warm up comes with it. Start by doing a little standing meditation, then stand on each foot. IIRC, 30 seconds is enough, but 60 or 75 seconds is a reasonable ambition. Anyway, do the move three times forward and backward on each side for this and each of the following versions. Then have your forarms parallel for the moves. (I'm pretty sure you can't do the move accurately with both arms that way.) Then do a mirror image version--you can't lean to both sides simultaneously, so you just lean backswards and forwards.
Here's a fourth variant I came up with. Do the move starting forwards with one hand and backwards with the other. I found that it's not difficult, but I bet it looks cool.