Like all the best visual illusions it’s conceptually simple but perceptually striking. In this case a falling ball seems to drop vertically when you look straight at it but seems to glide away at an angle when you see it in your peripheral vision.
Rather nicely, you can switch between the two effects just by looking back and forth. Make sure you click on the ‘Reversal’ button as well for a free-wheeling alternative version.
Visual illusions: the scooby snacks of perceptual psychology.