From the Super Mario Wiki
How to use the Yoo Who Cannon
Yoo Who Cannon is a Special Attack that appears in Mario & Luigi: Bowser's Inside Story. Its name is a pun on the interjection "Yoohoo!" and the word "Who?".
Mario and Luigi using the Yoo Who Cannon.
When using this attack, Mario and Luigi will briefly appear in eight barrels on screen; the player has to remember where they are. After a barrel rises, the player has to press the or button, depending on which brother was sitting there but, the player has to press that button before the fuse reaches the barrel to damage the enemy. If the wrong button is pressed or if the player takes too long, the other barrels will be destroyed, and it will do less damage. It costs seven SP to use this attack. After teaching Mario how to use Blue Shells, the Blue Paratroopas will give Mario and Luigi the 10 Attack Pieces required to use this move. It, Green Shell and Snack Basket are the only Attack Pieces that are required for the game.
This move is available for use in the Challenge Node. It becomes trickier when remembering the order becomes shorter.
Yoo Who Cannons were originally going to appear in Mario & Luigi: Dream Team, but was scrapped in the final version.
Names in other languages
Bunshin Taru Taihou
|Bunshin Barrel Cannon. Bunshin is an expression meaning "to divide and advance."
||Yoo Who Cannon
||Mega Human Barrel
||"Barile" (Barrel),"Cannone" (Cannon)
||Pun of "Butoi" (Barrel) and "Whoohoo" (truly written in Romanian as "Uuhu")