That trick is done by the 50% rule.. Make a white square, make a gray square that is 50% darker than the white one, then cast a shadow over these that is also set at 50% darker. the white square in the shadow will be the same as the dark square in the light. the dark square in the shadow will also be 50% darker and therefore create the illusion that there is a shadow. add a soft edge to the shadow and it is even more convincing.
It is a neat effect though. Escher would be proud.
Cheers!
