Begin at any point on track:

Hold [Shift] + [Option] and click on a track position before the race
begins. Note: This will start the race at "-1" laps.

Turbo boost:

Quickly shift into first gear when the bike is in mid-air while the
front wheel is higher than the rear wheel. Keep the throttle up as
the bike returns to the ground. Note: The rear wheel must hit the
ground first for the boost