On the level 'lethal highway'(level after doing hero on westopolis) rush through the level with sonic as your companion, but still try to attack evil kinevil spaceship, but rush. Hit the checkpoint at which sonic says 'I thought we lost em... etc etc' and approach the door. Occasionally, you'll see black, and if you try to walk past the door, then you'll fall. Move across using the magical 'floating' blobs.