From the Super Mario Wiki
The Shark-Bite Save is a Defensive Power Shot used in Mario Tennis: Power Tour exclusively by Clay and Ace. To unlock the move, Clay or Ace must receive at least 8 Speed, 8 Response and 4 Body PS Points. Once the move is used, the court will become made of water, and the ball will freeze in position. The user will then plunge underwater, swim off-screen and then swim back to hit the ball as a Shark, meaning that the user will have of moved position. The ball, once hit, will be returned as a Drop Shot.
"Turn into a shark and bite down on the ball."