judging by the fact that its a perfect circle, it would have to be something mechanical. Whatever it is it is very cool! He has some sort of mechanical device that helps him achieve it and he wont spill the beans!

http://www.flickr.com/photos/biskitb...57622454865956

looking at that one - notice the rock covering the bottom left of it, so it is something stationary that isn't done from behind the camera.