What to see
In the neighbouring checkerboard you can see a central “bulge”. This bulge is illusionary: the checkerboard is fully regular and each check is square, as becomes obvious when the small ticks are not present (use the "No ticks" setting in the pop-up menu preset to "Default".
Akiyoshi Kitaoka classifies the phenomenon as a “geometrical illusion with 3D impression”, I couldn't agree more.
The present demonstration indeed was inspired by his “Bulge”.
What to do
You can use the pop-up menue, the colour pickers, or the #checks stepper to try out various combinations.
First you might like to flip through the presets (below “default”) using the pop-up menu at top right. The “no ticks” setting shows the raw checkerboard. “Low contrast” does not impair the 3D impression. Interestingly, with the right relative lightness, settings one can change the bulge into a “pinch”.
How does it work? I do not know… But it's probably the same mechanism at play as in “Skye's Oblique Grating".