From the Super Mario Wiki
The Thunder Cast Shot is Wario's Offensive Power Shot in Mario Power Tennis. Wario pulls out a machine with a small Hammer attached and hits the ball. This looks like a normal shot, but it is actually powerful and leaves the opponent shocked. When shocked, the opponent has two seconds for reaction before a shock-wave hinders his or her shot. The shock goes away after about three shock-waves. The shot also causes the opponent to lose a Power Shot they may have stored. Recovery time is somewhat slow.
The Hammer machine was seen during the cut-scene of Bowser training Wario and Waluigi.