If you like getting scratched on your back do you lean in to it?
It's no different really. If it feels good, then lean into it hoping you get more. This includes cat/dog chin scratches and side of the face rubs. If a cat is getting back rubs while laying on its side, it will push out the hind feet hoping to catch on to something so that it has leverage to push into the hands.
I could be wrong, but I think there's more to it than that. I think it depends where you pet them. Petting them near their backsides seems to trigger that response in my experience. It makes no difference whether the cat is spayed/neutered either, I think it's their reaction to stimulation in that area.