From the Super Mario Wiki
The Piranha Catch is Petey Piranha's Sliding Catch in Mario Super Sluggers. After pressing the A button, Petey stretches out his neck, thus putting his mouth (which is how he makes a catch) near the ball and attempts to make the catch. This will make the sound that other characters make if they have a super dive. If the ball is caught, the Umpire yells "Out!", and the Nice Play is shown atop the screen. If the ball is caught as a grounder and it is thrown to a baseman/basewoman with a runner approaching, only then will the ump shout "Out!", and shown at the top of the screen.