There's a bit of a trick here... if you see her spinning one way or the other, then that's what you get, you can't see both at the same time. Basically your eyes/brain has a sampling rate that can change slightly to see one image or the other. The only way to change is to start sampling the images in the reverse order. I have no idea how to do that consciously. It's like watching wheels on a car start to spin "backwards."