From the Super Mario Wiki
The Backward Barrel is an item collected from a Blue Item Balloon in Donkey Kong Barrel Blast.
The player throws the barrel backwards as an obstacle when using this item. It then turns into three barrels for more chances of hitting someone from behind. If used by a Kremling, they turn into TNT Barrels. Neither barrel can be destroyed by attacking it or running into it while using a Wild Move. Both barrels will also make a player lose ten bananas when colliding with one.