Rotating Sparge Arm, how does it work?

I keep seeing these rotating sparge arms but I'm not sure how they actually work? Do they spin from the water flow? www.northernbrewer.com has a couple they sell and I have seen them other places as well.

Does anyone know?

The water flows through the arm via gravity. I'm sure there's a bladed thing inside that makes it twist around. Just based on water-flow.

Yes the water flows thru it and there are tiny holes in the arm that the water shoots thru and that causes it to spin. Like a water sprinkler for your yard.

Thanks guys, I assumed that is what happened, I was having a hard time visualizing it.

It's one of the best things you can purchase for your all-grain setup! Combined with a ball valve, you have an on/off switch for your sparging, and you can adjust it to the perfect amount of flow. Just be sure to keep your sparge water fully above your mash/lauter tun so that gravity can do its job for you. The perfect setup is to pre-heat your water and store it in a cooler with a ball valve, and elevate this above your mash/sparge cooler.