From the Super Mario Wiki
The Summoner Shot is an offensive power shot in Mario Tennis: Power Tour. It is used by Willy in the game's story mode, and can be unlocked for usage by the player by having the following PS Stats: 7 Response, 7 Muscle and 5 Balance. Once activated, the user will spin on the spot while an emblem encases the tennis ball. They will then hit the ball, which will fly in a strong curve. If an opponent returns the Summoner Shot, they will be encased by the emblem, and have their movement restricted.
"A shot that confuses opponents with a magic circle."