Well maybe not everyone will agree with you, but like you say the very perception of an object in motion:
Originally Posted by IDDrummer
clearly has to do with the different perceptive tendencies people have.
Nonetheless, the fact remains that using her outline (curves of her back, her ponytail, breats,...) you can figure out which direction she is pointing at any given time in a cycle, and piecing these directions together you can figure out the rotations.
I too first thought that I was changing it at will until I found out that it was coincidently changing right when I thought I was at my AHA focal point.