Why is it that people "spin" to stay lucid? I've seen so much reference to the method but I don't really understand why it's effective.

I ran a search on the forums and throughout the many results I only found experienced oneironauts speaking of doing this to stay lucid - never a direct explanation.

I haven't had a lucid dream yet so I can't begin to relate this to any of my experiences, any information would be appreciated, thanks.