From the Super Mario Wiki
A player is randomly picked, and they are given a bomb in the shape of the Wii Remote. The bomb holder then plays a microgame, and if they succeed, they pick the next player to play a microgame and which form they will use; each form can only be used once in a round. However, should they take too long to pick the player or the form, then the bomb will explode and they will lose the game. The bomb will also explode should the player lose their microgame.