Writing on Mindhacks, Vaugn Bell—perhaps one of the best neuroscience writers around—explains the trick:
"If you don't believe me, try this simple, but startling demonstration. First, hold your eyeball perfectly still. You could use calipers to do this, or a drug that paralyses the eye muscles, but my favourite method is to use my thumb and index finger. Using the sides of your thumb and finger, press on the bone of the eye socket, through your upper and lower eyelids. Do this gently. Try it with one eye first, closing the other eye or covering it with your hand.
"With your eye fixed in position, keep your head still and soon you will experience the strangest thing. (You will have to stop reading at this point. I don't mind. We will pick up when you have finished). After a few seconds the world in front of you will fade away. As long as you are holding your eyeball perfectly still, you will very quickly discover that you can see nothing at all. Blink, or move your head, let go of your eye and the world will come back."
What's happening? Essentially, your eye is casting the exact same pattern of light on to the retina. Because nothing's changing, the receptors in your eye start sending messages to the brain less often, so the brain is passed information less regularly. Gradually, that means the image fades to nothing. Put crudely, it's like bad smells that disappear when you're exposed to them for more than a few minutes.
