As soon as the animation of the cars pulling up to the line is over hold A and hit up then down on the thumb stick. Let go of A and slam the R trigger down to get you motor rev'ing.

If you've done the trick right you'll be put "back on the track" about 10 feet in front of the three other cars, and can take off at full speed. You'll get a head start bonus, and more importantly a GREAT head start!

This trick only works on U.R.L., Sprint, and Circut races. It will put you off to the side if you are in the first starting position.