Once the game begins, a camera object moves from the top of the 'first' page to the top of the 'second' page. At this point, all objects are moved up the bitmap by one display length. Any objects that are now off the top of the bitmap are removed from the game, and new obstacles are created for the new 'third' page. The camera is then repositioned to the top of the first page. This process will be repeated until the game ends.
Here's a YouTube clip to demonstrate: