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. It will temporarily slow down anyone who manages to return the ball. 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. Returning a lob with this power shot when its at the highest point in the air will make the ball fly over small characters, making them unable to return it.