This is really cool. Stare at it for a while and you should be able figure out why (no, it's not because her nipples are inexplicably fully rendered).
Explanation and a neat trick after the jump.
The image is an optical illusion. The girl appears to be spinning in one direction when you first look at the image, but after a while she appears to switch directions.
The great thing is, you can actually make her switch directions in your mind at will. The trick, at least for me, is to focus on the leg that's raised off the ground. Think of it as the girl's right leg to make her spin clockwise, and as her left leg to make her spin counterclockwise. You could also focus on the bent arm or the hip of the raised leg to make this happen, but I prefer, for reasons I don't think I need to explain, to focus on the leg. If you get good at it you can actually convince yourself that she's just swinging back and forth, and not spinning at all.
Also, she appears to spin faster on Firefox than IE, which makes it harder to control her direction. If you download the image and open it in an image viewer that can read animated GIFs you should get the slowed-down version.
EDIT: Here's a visual aid.
Frame with raised leg pictured as right leg:
Frame with raised leg pictured as left leg:
The black is the same in all three pictures.