Spinning Shell Dash
From the Super Mario Wiki
Spinning Shell Dash (not to be confused with Water Shell Dash) is the Defensive Power Shot used by Bowser in Mario Power Tennis. It involves Bowser getting inside his shell, spinning over to the ball and hitting it. The shot is similar to his Whirling Fortress attack in the Super Smash Bros. series.
This power shot only executes if the ball is at ground level. It will not activate if the ball is too high in the air.