From the Super Mario Wiki
Deep Breathing is the Wii Fit Trainer's down special move in Super Smash Bros. for Nintendo 3DS / Wii U. The trainer inhales, a red circle appearing around her and a blue ring getting closer; the player must press the special attack button when the two align, at which point the trainer recovers damage and the power of her attacks are increased; if the player fails then nothing happens. In addition, failing when using the move after receiving the power boost immediately removes it. If the move is used immediately after the ring closes in more slowly, though it gets faster the longer the player waits after using the move.
There are two custom unlockable variants for this move. The first variant is Volatile Breathing, which no longer heals or powers up, instead creating an explosion that damages nearby opponents, dealing more damage when closer to the trainer. The second is Steady Breathing, which is faster to execute and gives the Wii Fit Trainer super armor for the entire duration of the effect rather than attack power, but it takes longer for the move to fully recharge; the ring will move more slowly than usual when using it immediately after using it once.