Flash Fabrica

Recently, I bought a book called "Gallop! (Scanimation Books) ".
The picture of animals starts to move when you open the book.
That is so amazing.
This is a Flash which is an accurate reproduction of the book. (motion-simulated)

Why does a horse run?

This Flash contains only two pictures.
①

②

What only moves is the slit.

In this animation, only the slit of ② is moving.
Why does the horse look like running.
Look at the Flash carefully and think.