From the Super Mario Wiki
The Flashflop is an offensive Power Shot in Mario Tennis: Power Tour. It is considered an enhanced version of the drop shot, as the ball will very weakly glaze over the net, and straight onto the ground behind the net. Upon the shot's activation, the user's racket will absorb energy and strike the ball, enveloping it in a yellow light for the duration of the shot, and if returned while the shot's graphical effects remain, they will become dizzy. The Flashflop Shot is unlocked by having a PS Balance stat of 3. In the story mode, the Flashflop is also used by Mynx.
"A drop shot that uses a bright light to blind opponents."