Maybe it's not working for you, but it works on my phone... The lines of which the cubes are drawn are white/black. By changing the amount of each on a side, (WHITEblack whiteBLACK BlackwhiTE blackWHITE etc) in quick succession, one edge of the cube can look like it's approaching or receding. It's based on the way shadows would behave for a real cube, and your brain processes it as such. (The ones I made up aren't in any order, whereas the ones in the gif are carefully synchronized)