Hypnosis Spiral

Why is it that whenever I rub my eyes, I see black and white spiral?

So my question exactly what it says, every time I rub eyes, I see the hypnotic black and white circles. About 4-5 of them at the same time. Seem http://www.hypnosis.com.au/images/stories/hypnosis-spirals/blackwhitespiral.gif if you know what I mean. I continue to see them until I stop rubbing your eyes. Oh, and the two eyes must be closed at the same time for me to see them. You can also get the same effect as if the press in both of my eyes. What is it and what causes it?

http://en.wikipedia.org/wiki/Phosphene pressure phosphenes

Hypnosis Spiral