Instructions
Race on a map that allows you to fall off the edge of the track, whether into water or into the bottomless expanse. "Rainbow Road" and "Wario's Gold Mine" are two maps notorious for such features.
Drive your kart off the edge and wait for the game to reset you onto the track. This will take a couple of seconds.
Press the "2" button the instant your kart appears back on the track. This will give you a sudden burst of extra speed, allowing you to catch up with the other players faster.