From the Super Mario Wiki
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The Bubble Save is a Defensive Power Shot that is used in the Game Boy Advance game Mario Tennis: Power Tour. In the games story mode, it is used by Barb, and opponent in the Island Open. It can be unlocked for Clay and Ace's usage by gaining L4 Spin and L4 Speed. Upon activation, the user will generate a large amount of bubbles, which will bring the ball towards the user, allowing them to return it.
"Blow bubbles to bring the ball in close enough to save."