From the Super Mario Wiki
The High-Power Shot is an Offensive Power Shot used in Mario Tennis: Power Tour. It requires L3 Muscle, and is used by Whisker, Barb and Chas, making it the most frequently used Offensive Power Shot and making it one of (along with the Quick Save, Running Save and Quick-Step Save) the most frequently used Power Shots in the game. Upon activation, the user will jump in front of the ball and hit it to the opposing side of the court, the struck ball leaving flames behind it whilst moving.
It can be considered an upgraded version of the Power Shot, as it shares its name, offers more power and pushes any opponent who returns this shot further back.
"An intense Power Shot with a little added oomph."