Hold 2 when the countdown reaches "2" and that number begins to fade.

While the timer to begin the race has just started, hold 1 + 2. As soon as it is about to say "Go", release both buttons and press 2 as you would normally do to start the race. If done correctly you will get very nice boost that will usually put you in first place while in the 50cc or 100cc classes. This may require some practice.