From the Super Mario Wiki, the Mario encyclopedia
Bonus Chance! is a minigame found in Yoshi Topsy-Turvy. After each mission, Yoshi falls from the ceiling and stomps on a wheel, causing it to spin. The wheel has eight spots, if the player get a Happiness Medal it has four of them with Yoshi's head and the other four with a green "X" icon. If the player get a Silver Medal, it has two Yoshi's head icons and six with a green "X" icon. Additionally, if the player gets a Gold Medal, he gets better odds at winning. The player must press to make the wheel stop spinning. If a Yoshi's head icon stop at a blue arrow, Yoshi can play a Bonus Mission!. If it lands on an "X", then he loses the game.