These boxes aren't moving – an optical illusion I can't explain

You've probably seen animated GIFs that blink between negative and positive images. They trick your brain into thinking the images are moving. That's the concept behind this GIF of two necker cubes, which has the added feature of arrows placed in the centers of the cubes. When the arrows point up, the cubes appear to move up. When the arrows point down, the cubes look like they are moving down. Other kinds of arrows that convey the idea of rotating or shrinking make the cubes look like they are rotating or shrinking.

I had to hold a piece of paper against the edge of the cubes to convince myself that the cubes weren't moving. But something else is going on, because if I cover the arrows with my fingers, I can still "see" the cubes moving in the way the arrows suggest. What am I missing?