/sciDuck here with some link:
http://theness(dizot)com/neurologicablog/?p=27If you no lika da sauce quick answer is lackluster: We live in 3D land, 2D images can trick us based on assumptions made by the visual cortex (changing size of foot = depth). We've never seen anything move like this without actually spinning so the brain makes it "spin" because that's what makes sense. Even though logically we know it isn't. The brain picks a direction pretty much at random, or perhaps based on experience.
I see her spinning the way I would spin were I doing a half-assed pirouette because of body mechanics. If I look away and look back she changes directions sometimes. The three images don't have to sync up because they're just different enough for your brain to delineate.
S me re son yo ju t r ad t is qu ckl . Or tihs for taht mtater.
It's a variation upon an old theme. If you're at uni ask anyone whose had psy101 (or the prof) for examples and answers to your questions. If you're high school has a psych class ask that teacher. If neither apply head to the library.
I wouldn't trust Wiki for (well, try Scholarpedia if you're really averse to going outside) psych because it's too contentious.