Now I want to know who designed the image and how they explain the mechanism.
I think it is to do with assuming direction to a black shape--the shape is two D but we assume solidity, so one half of the brain assumes a certain spin....
...uh oh...I just thought of quantum mechanics and that means I'll be leapt upon by left-brain types. Don't fight forces, use them R. Buckminster Fuller.
I've just decided I'm ambicerebral. If that's a word... "Pretending that you already know the answer when you don't is not actually very helpful." ~Migeru.