Quite simply, when you start a race, rewind doesn’t become available before roughly the first corner, and disables itself roughly at the last corner. The suggestion is to not do that, and that rewind should extend to the entire time the player is in control of the car, from the end of the start countdown, all the way to the finish line.
And I will insist on that last part, as long as you haven’t crossed the finish line, you should be able to use rewind. It’s a little bit absurd to have this driving aid that exists entirely to let players correct their driving mistakes, only to have it fizzle out for no discernible reason if they make a mistake right at the end.