In Skater Island, open the beach area by grinding on the pirate flag. Then, skate to the ramp shown in the picture that is at the right of the entrance. Hold down the ollie button (A for the Gamecube) and skate up the ramp. Ollie as high as you can (release the button) so you travel high up onto the hill behind the ramp. Don't do many tricks, but we've been able to do this while doing a Boneless. If you're lucky, you will crash back down into the ramp and land upside down, stuck in the ramp. There is no way out unless you restart the run. We had trouble doing this in Free Skate mode, but could do it easily in 2-player mode. Sometimes you'll also get stuck halfway through the hill. It's interesting stuff.