I own a book, "the magical eye". Long time ago I was very impressed by that strange "3D" effects.
It's hard to describe "how to fo it":
Do not focus on the picture with your eyes, no picture is recognisable in the chaos of dots and colours. Relax and look at the picture. It takes a little time, the right angle and light are important. Try to get into a ‘trance’ while looking at the picture. The image is created in the brain and then stands out from the chaotic background in 3D. One small disturbance and the image in your head is gone again. It is difficult to describe.
Nice one, Johan!