From the Super Mario Wiki
Backwards Bill is a Bullet Bill item that players can obtain in the Perilous Palace Path board in Mario Party: Island Tour. Players have a chance of randomly obtaining it from Item Spaces or from winning minigames and selecting the item. It serves as a more powerful variant of the Setback Shell, where selected opponents get knocked back 5 spaces rather than 3. Its opposite counterpart is the Golden Dash Mushroom, which helps users gain five spaces.