From the Super Mario Wiki, the Mario encyclopedia
The Superswift Save is a Defensive Power Shot that appears in the game Mario Tennis: Power Tour. This Power Shot is used by Chas and Dweezil and can also be used by Clay or Ace. In order to obtain it, Clay or Ace must reach 3 PS Speed points, which are obtained by playing the Treadmill and Duck Walk mini games. Upon activation, the user while lunge for the ball, in a similar fashion to a normal lunge or the Running Save. This shot can be considered an enhanced version of the Running Save, as evidenced by it's description and similar effects.
"An enhanced Running Save that can reach distant balls."