Sometimes, when I down-b on accident I...
No srsly though, transformation is all laggy and slow now, I think it is a real oversight on the developer's part. It make transforming very risky. In theory, sheik could be fast and combo to build up damage, while Zelda has KO power, in addition to be an effective combination through clearing the weakened states of attacks on transform. However, it doesn't really work that way, because both Zelda and Sheik cover all of their needed elements of damage, projectiles, and KO power separately, or at least it is clearly true for Zelda, and they have no need to rely on the other. What it COULD be useful for, however, is if it is found that Zelda and Sheik have individual matchups which they excel at, switching can allow "on the fly" character counterpicking.