This glitch is done in two parts.
In order to perform this glitch, you must first activate the cheat code that turns all power ups to their maximum power. (The reason you need this is that you normally can't get a maximum level powerup on the Testing Grounds.) To do this, create a racer and name him mxpmx. then, with the cheat activated, play the testing grounds with any racer. (it does not have to be mxpmx)
Drive out of the starting box and get a speed power up. (it is the green lego brick) The cheat code will bring it up to its maximum level: Warp speed. Now, speed up to a wall and right before you hit it, use your power up. You will enter a wormhole (that is normal. the power up is supposed to do it) If you hold the up arrow down while you are in the wormhole, you will wind up outside of the course.
The area you wind up in is a gradient yellow fading into blue at the bottom. That is the backround that serves as the sky. When you arrive, you will immedianlty begin heading backwards, leaving an imprint of your car on the backround wherever you go. You can still use the arrow keys to influence your movement, but you can't stop the backwards movement. When you reach the bottom of your monitor, your car will pop back up onto the track with a puff of smoke.
However, there is a small chance that something else will happen. If done in a very particular way, your car will end up stuck in the wall that separates the track from the area outside the map. For perhaps a minute, nothing else will happen except for a the sound effect for bumping into a wall playing consistantly. After a certain amount of time, though, your car will suddenly shoot out of the wall, and "fly" across the top of the map for a bit at high speeds. It takes at least a good seven seconds for your car to return to the ground and stop completely. This alternative is extremely difficult and will usually happen by chance.