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 himself/herself) 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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