i'm not exactly sure why it works, i think it was something like, your mind works like a camera, so when you look at it and your eye shifts its vision to (or just focuses on) another portion of the image, your mind tries to smoothly transition from the image of 'your first glance' and the image of 'your second glance'. thus, the image moves to the side, but the circular arrangement of the dots makes them appear to rotate, rather than merely shift.
but, like i said, i'm not really sure :l