Wheelies are a stunt that can be pulled off when a character is driving very fast and the Wii Remote is shaken violently.
Performing a wheelie differs between the weight/size of a character. A light character can pull of a wheelie very easily, but there is a 50% chance it will be very weak. An average character cannot pull off a wheelie as much as a light character, but the wheelies are much more powerful. It is hard to pull off a wheelie with a heavy character, but the wheelies are extremely powerful and give the kart a large speed boost.