I have searched in the N900 documentation and on the web, yet, I have not found a description for this neat way to switch application menu screens. Tapping the available application screen at the outside (left, top, right, bottom) will switch back to the desktop. If you are in available application screen two (you pressed the More... icon), tapping the screen left, top, right or bottom will take you back to the first application screen. Another tap, and you are back at the desktop.