Transform is the down special move of both Zelda and Sheik in Super Smash Bros. Melee and Super Smash Bros. Brawl. Using the move transforms Zelda into Sheik and vice versa. In Super Smash Bros. Melee the transformation is faster as both characters are loaded at all times; in Super Smash Bros. Brawl, however, the transformation is longer since the game needs to load the other character separately mid-battle. Taking damage while the move is in progress immediately cancels the transformation. In the latter game, Zelda or Sheik can use Transform at the moment Kirby activates his Cook Kirby Final Smash to avoid being caught.
In Super Smash Bros. for Nintendo 3DS / Wii U, mid-battle transformations are removed from the game, and Zelda and Sheik are two separate characters. As such, both characters have been given a new down special move: Zelda's new down special move is Phantom Slash, while Sheik's is Bouncing Fish.
The move is based on The Legend of Zelda: Ocarina of Time where Sheik reveals to have been Zelda in disguise all along. Using the power of the Triforce of Wisdom, she is able to physically change her appearance, allowing her to disguise herself as a male. The sound that plays when Zelda or Sheik transforms is the traditional Zelda "secret revealed" chime.