It is always spinning in one direction. It's your perception of the image that changes, not the direction. If you don't believe me, then watch it with a friend. You'll both see it rotate in different directions, occasionally.

I notice that it switches direction when you watch the reflection of the feet, then look back at the body. I think it works by tricking your brain into alternating the rotation by not giving you a point of reference. The same thing happens when you watch a vehicle's wheels on the highway; sometimes it looks like they are spinning in the opposite direction than they really are.