Ive only done this in a Cobalt and Gallardo and most of the time I was in a pursuit when it happened so I don't know if it works on other cars or race modes. Anyway, what you do is you have push the R3 button forward (and maybe push it like a button), X button, and square button in certain order and/or a certain amount of times. Trust me it works. You'll have to
fool around with the three buttons a little bit before it works. After that is done, your car should be accerlating by itself until you either brake, or smash into something.