Transform
Transform is Zelda and Sheik's Down B move. It allows them to transform into each other. If one is quick enough, one can interrupt the transformation in its first frames of animation.
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/Sheik transform is the traditional Zelda "secret revealed" chime.
In Brawl, the animation for the transformation takes signficantly longer, but Zelda/Sheik is invincible during the majority of the transformation. This is most likely due to a software limitation, not an intentional change, as the transformation time is the same when Zelda/Sheik's reaction time is slowed down by an item, assist trophy, or practice mode. Additionally, Zelda/Sheik's downwards momentum is slowed down so one can technically still use the transform from Sheik into Zelda to recover. It is possible to use this move to dodge Final Smashes such as Cook Kirby.