Good Lord! Actually, when I was a kid, we had a book full of old etchings, and it came with a piece of transparent plastic covered in fine lines--when you moved it over the pictures, it gave them the strange illusion of movement. Doing the same thing with these images creates a strange, hallucinogenic effect as the skin on those faces literally crawls all over the place. Bizarre!I hope that made sense.
I walk around with such transparent plastic on my glasses. (This might explain something.)