It's pretty easy when you remember that vision is based on contrast.
Fun trick: Hold your left palm flat in front of the area left of the A/B tiles. Hold your right hand as a fist over the right half, like you are gonna give a thumbs-up. Now, slowly extend your middle finger like you're going to sideways flip your screen off, so the finger covers the gap between the A and B tiles.
As that last bit of the light tile is covered and revealed, you will sense your perception of B darken and lighten.
