Look at this Youtube clip. In the middle you have the ambiguous dancer. The one on the left has some cues added so you make her spin in one direction. The one on the right the same, but in the opposite direction.

Now do this. Cover the one on the left with your hand. Now the one that is only a shadow will spin in the same direction as the right one. Cover the right one. Now the middle one will spin in the same direction as the left one.

Clever huh?