If Mario arrives in a galaxy while the Cosmic Comet is circling it in "Super Mario Galaxy," the mysterious Cosmic Mario will appear and challenge him to a race. Winning the race nets Mario a Power Star, but losing the race will cost him a life. If Luigi arrives in a galaxy circled by the Cosmic Comet, he will have to race Cosmic Luigi, an even faster opponent. Fortunately, timing your start just right can give you a boost at the beginning of the race. This trick only works against Cosmic Mario and Luigi, not against Spooky Speedster or other racing opponents.
To get a starting boost at the beginning of a cosmic race, hold up on the joystick. When the timer counts down to two, hold down the "Z" button. Just as the race starts, press the "A" button. It may take a few tries to get your timing just right, but if successful you'll be rewarded by a speed increase. A yellow glow will appear around Mario or Luigi to indicate the boost.