Select to any track and drive at a high speed (about 200 to 270 mph). Ram the car at any wall with arrows. If done correctly, you car should fly off the tracks and into the air. You will end up back on the track if you go too far.
You can unlock cars by completing Arcade Mode. Also, a 4x4 and a stunt car can be unlocked by going through every route on Arcade Mode. A stock-type car can be unlocked by beating all the "best lap" ghosts on Time Trial Mode. You can also unlock more by the following.
Complete Championship Mode in 1st place point value, and beat 21st track in 2nd place.
Beat the "Best Lap" ghost on Time Trial once
Complete Championship mode in 1st place point value, and beat 21st track in 1st place.