The Power Card has the ability to endow its user with superhuman strength, enough to lift tons of weight. Power would be invisible, showing off its strength, until someone challenges it and tells it what the challenge is. If the challenger wins, Power would surrender itself, making it able to be sealed. It is possible that Power is the sister card to The Fight, much like The Big and The Little, as they both have similar abilities and personalities. Power's true form is a small (probably not much taller than 2 feet), elfish girl wearing a circus-like costume with a pink color palette and three, pink circles on her forehead. It is under The Firey.
The Power first appears in Episode 13. The Power was breaking open the animal cages at the Tomoeda Zoo and earlier, overturned the penguin slide at King Penguin Park. When it started to torment the elephants, Sakura challenged The Power to a game of tug-of-war. But even with the elephants' help, Sakura is slowly losing, so Syaoran uses The Time Card to trick Power into thinking it had lost. Since it 'lost' the match, it surrendered itself to Sakura so she could seal it. Sakura used Power later that episode to return the penguin slide to its original, upright position.
The Power reappeared in Episode 20, where Sakura used it to power up her Sealing Wand so she stood a chance against The Fight. It didn't help much, except when Sakura randomly swung her wand and it smacked Fight right in the head, probably causing one of the most painful card captures of all the Clow Cards.
The Power was transformed into a Sakura Card in Episode 52. In another of Eriol's trials, he messed up King Penguin Park by creating a giant hole and upturning the penguin slide. Sakura used The Power to put the slide back to its rightful position.