The Whirling Fortress is an attack performed by Bowser used in the Super Smash Bros. series, originating in the GameCube title Super Smash Bros. Melee and later appearing in Super Smash Bros. Brawl. Bowser would perform the attack when the player pressed B in addition to pressing up on the josyick. The move would cause Bowser to spin in the air, and thus is considered a recovery move. While spinning, he'll hide in his shell.
The move can not only be used to get back onto a stage, but can also be used to dramatically harm one's opponent.