At the start of a race, hold [Up] a moment after the countdown meter reaches '1'.
Missile Speed Boost:
You know when you get hit by a missile, your character flies upwards and then hits the ground. At the moment you land, hold up and you'll boost as if you used a Red Nitro.
After getting hit by a missile and your character flies up, hold [Up] at the exact moment you land back on the ground to get a speed boost.