Video: I'm not blind, I can totally see that the square is moving. First, it tilts to the right side and then it tilts to the left side, like a seesaw of sorts. But it's actually a mind-bending illusion that makes you think it's moving. The parts that make up the lines are actually just "drifting", as in they seem to be just cycling colours in a certain direction. That drifting makes it seem as if the whole shape is moving, but the bigger part actually remains still.
Never trust your eyes.