From the Super Mario Wiki
A Shell Shock is an item in Super Paper Mario. Its design comes from the 8-bit sprite of a Green Koopa's shell from the game, Super Mario Bros. Its name comes from the state of stunned distress caused by constant exposure to active combat often characterized as "shell shock".
When the player kicks the shell, it ricochets away and rams into enemies. 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.