The ball flip is a trick which requires a yoga or exercise ball. It involves jumping onto the ball, and rolling/flipping off backwards into a backflip, and landing back on your feet. The move looks neatest when the backflip off the ball is a layout.
The move seems to have been first popularised by James Stewart, who released a video on Youtube of him performing the trick in August 2007. An animated gif of the trick also reached the front page of Digg with nearly 10,000 diggs.
James's Youtube channel also features a clip of him performing the trick live on-stage when the ball pops causing him to land on his head.