He keeps sliding down his pants, as more and more of the shaft appears, until the head comes into site. Once his cock is free of its restraint, it bobs out into the open and, perhaps, begins to harden.
It's like opening a present as more and more is revealed until you see the whole wonderful gift. Thinking about this is making me hard right now. How about you?
Most of these images are much larger than they appear here, so here is a free folder containing all of them.