How do you do a boost in Mario Kart wii?

At the start of the race, before you go, 3 2 1 flashes on your screen. The second that the 2 disappears, hold down either 2 or A, depending on if you are playing with the nunchuk. You will get the boost of speed. But be careful; if you hold down 2 or A too early, your kart will smoke and spin out.