@sjw
It has to do with how the lines change color between white and black. If you manage a good freeze frame, you might notices that one cube's black lines are slightly shaded white on top, while the other cube's black likes are slightly shaded white on bottom. Or left and right, and so on. It's things like that that create the illusion. Slight shading that changes in time is what tricks your brain into seeing movement.

I've seen something similar years ago, but it was using circles instead.