From the Super Mario Wiki
Rainbow Flutter is the Offensive Power Shot used by Yoshi in Mario Power Tennis. Yoshi performs a Flutter Jump in the air and then hits the ball, which leaves a rainbow trail behind it. This may be the inspiration for the Flutter Dunk from Mario Hoops 3-on-3.
This power shot is able to hit the ball even if it's very high in the air, and is in fact the only Offensive Power Shot capable of doing this.