See the runner - grab onto the black stripes on the right and slowly drag it to the left. This was billed as an 'optical illusion' - not sure if that's apropos. You're not being 'tricked' into seeing anything. It's as much an 'illusion' as regular frame by frame movies give the 'illusion' of motion.
Thanks to the Bad Astronomy Blog for this one.
I've seen a children's book that has that exact same optical trick: the page has the slatted background image, and there's a plastic sheet with the fence post slits on it.
Move the plastic from side to side and you see an animation of a horse running, or a dog or cat running (this example was the cat), or a bird flying, etc.
Actually, now that I pay attention, the web page links to the book I'm thinking of: Gallop!: A Scanimation Picture Book.