From the Super Mario Wiki
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When the player kicks the shell, it ricochets away and rams into enemies, damaging them. But it will also hurt the player, so they must be careful. Using the item activates a minigame where the shell bounces around the screen, and the player must press while hovering over it with a crosshair. If the player succeeds, the shell grows to almost three times its original size, but if the player fails, it stays small.