A charged shot that belongs to one character, not a generic smash, is the gimmick in Mario Power Tennis. Camelot added those special shots to the Mario tennis formula in this 2004 GameCube game, so Peach's power is not Mario's fire.
Gimmick courts tilt, speed up, or flood, and a pair of players can trigger the charge only after a rally earns it. Bowser's shot and Waluigi's trick are comedy as well as points.
Camelot had already made a Mario tennis game without the power system. The charged shots and the strange courts are what the "power" in the title adds.