From the Super Mario Wiki
It made its debut in the Battle Mode found in the main menu of Super Mario Bros. 3 (Super Mario All-Stars version), where it would reverse the two players' positions on the screen or reverse their super/small forms.
It also appears in Mario Party 3. It is green with a red U-turn symbol on it and is Donkey Kong's favorite item. The player who has it can make any player on the board (including themselves) go in reverse when they are moving across it, until they reach a fork or area with multiple paths, then the person inflicted can choose to go forward in another direction, or still go backwards.
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