From the Super Mario Wiki
Energy Ball is the Offensive Power Shot used by Paratroopa in Mario Power Tennis. It involves Paratroopa flying backwards, then spinning forwards again, igniting the ball with a shiny purple energy and making the stage darker. When hit, it goes high in the air similar to a lob shot. It sends players who hit it spinning in a tornado, making them unable to move for a few split seconds. It can be countered with Yoshi's Rainbow Flutter or a high range defensive shot like Donkey Kong's Boomerang Banana Return or Diddy Kong's Barrel Jet.