You'll need a Mushroom for this glitch to work, which makes it very useful in Time Trials. From the starting line, back up some and line up with the first tree to the left you see. Approach it and get next to it, lining your racer up so the angle between you and the wall in front of you is about 30-40 degrees (which varies upon the racer). Back up to the center line of the road without turning. Then accelerate. Pop a wheelie and, as you start it, use a Mushroom. You should be launched over the wall to the second half of the track. Back up into the tunnel and hit a wall in there, then you can finish the lap as normal.
