From the Super Mario Wiki
Ads keep the MarioWiki independent and free :)
The Propeller Mushroom (also known as Propellerhead or Propeller Suit) is an item which was introduced to the Mario franchise in New Super Mario Bros. Wii. As the name implies, the item is used to let the player fly through the air, spinning like a propeller until gently floating to the ground. Descent can be delayed if the player shakes the Wii Remote while falling. If left stationary, the Propeller Mushroom will fly out of the stage. In order to activate the propeller, the player must shake the Wii Remote. Holding the down button will make the player drill down for an attack with the same effects as a Ground Pound. Propeller flight is not continuous, but after the player lands, they will be able to take flight again. The Propeller Mushrooms's appearance is an orange-capped Mushroom with two light-yellow spots and a yellow propeller atop the cap. Obtaining a Propeller Mushroom turns the player into their Propeller form, adding a jumpsuit (in color of the character) with a large helmet-like cap that has a propeller sticking out of it and the player's emblem emblazoned on its sides. It debuts in World 1-1, and it is one of the main powerups of the game, alongside the Penguin Suit. Propeller Blocks and Propeller Boxes have a similar effect.
A player can grab onto another user who currently has the Propeller Mushroom. To do this move, the player must hold and shake the Wii Remote. Then the player that is holding on to the other player can shake the Wii Remote again, making both of them fly. If the player shakes the Wii Remote when someone else is holding on, the player that was holding on will fall off. If the player gets swallowed by a Yoshi, they can shake the Wii Remote and fly out of the Yoshi's mouth.
This item reappears in New Super Mario Bros. U acting in the same manner as in the Wii version, but the object can only be found in a Toad House from Superstar Road. A slight change is that the power-up in New Super Mario Bros. U cannot spin around as fast when the player shakes the controller, when compared to New Super Mario Bros. Wii. In New Super Mario Bros. U Deluxe the Propeller Mushroom acts the same, but can now be obtained from Enemy Courses.
Names in other languages