The Back Space (also known as a Reverse Space) is a space that first appears in Mario Party 3.
Mario Party 3
In this game, Reverse Spaces only appear during Duel Mode boards. Landing on one of these will send the player backwards and won't be able to make any direct turns. However, if the player going in reverse has a partner behind them and runs into another player, then a battle will occur with the partner in behind being able to attack the other player. Landing on a Reverse Space on Backtrack will actually change the direction of the entire board.