From the Super Mario Wiki
Boo Blast is the Offensive Power Shot used by Boo in Mario Power Tennis. Boo manages to spin around, lifts up its racquet, while several tiny Boos go into its racquet to become into a bigger boo attached to the object which forms a Hammer. Once Boo hits the ball, all the Boos then follow the ball and harass the receiver. In the case the receiver hits the ball, he or she will be stunned briefly and disoriented, making him or her difficult to control for a time.