Hard to explain because I'm retarded, but basically there's multiple patterns that make a separate cat image. The sheet of plastic being moved is constructed in a way that only one pattern is shown at a time. Moving the the plastic is like watching a repeated reel those old projectors in old movies that used that plastic with a ton of images under each other.
Alright, im no illusionexplain, but i think (keyword think) i know how this works.
Its a series of pictures of the cat in different stages of running, but cut up into hundreds of little lines, so as a screen like this : [lllllll] goes over it, it blocks out everything but one scene, so as it moves across it covers the other scenes and shows one, then covers the others and shows the next, and so on and so forth.