Barrel Cannon Blast
From the Super Mario Wiki
Barrel Cannon Blast is the Offensive Power Shot used by Donkey Kong in Mario Power Tennis and Mario Tennis: Power Tour. It involves Donkey Kong placing a cannon behind him, flipping into it and then shooting the ball back with a force that sends the receiver skidding backwards. It is easier to hit back the ball when the special shot is used if the player is a distance away from Donkey Kong.