What to see?
You will see a ball spinning a certain trajectory on an plane plan made out of grids. The grids have the purpose to make the illusion happen and in the same time to make us understand how it’s made.
What to do?
You have to pay attention at the trajectory of the ball and its shadows.
In the first place the ball has no shadows and it will probably seem that it’s moving in a plane and simple way. Does it seem to rise above the floor of the checkerboard? Does it seem to move in an odd way? Now! If you want to see something different you have to change the shadow of the ball.
When the ball has a diagonal shadow it will seem that the ball rolls on the floor. When it has a horizontal shadow you can easily see that the ball actually rises from the checkerboard; and we combine the two shadows you can see the ball bouncing.
The truth is that the ball never changes its trajectory and remains on the same path no matter what shadow appears. Nice trick a shadow and a human mind showed us!
How it works?
Because the ball has no shadow we perceive its trajectory as being the same in depth and in distance. This happens because our visual system can’t decide between the two simple moving possibilities. Our visual system can perceive the ball either as rolling diagonally across the checkerboard, either rising diagonally across the same plane. In front of this situation the brain tries to find the easiest way to resolve the ambiguity of the missing shadow.
In our case when the shadow appears we have a reference point for the movement of the ball. When we combine them a strange thing happens: the ball seems to bounce along the floor, because the ball appears to move in different directions. But when we take out all of the shadows our brain rejects the idea of a different motion that the one we talked about at the beginning.
So in the normal life, our system relays on the shadows of the environment which can give it reference points in interpreting or analyzing what it perceives.