From the Super Mario Wiki
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The Spider Save is a Defensive Power Shot that Clay and Ace can use in Mario Tennis: Power Tour for the Game Boy Advance. In order for then to learn it, L5 Response and L4 Balance must be acquired. Upon activation, the user will generate a spider web that will catch the ball. It will then make the ball bounce back towards the player, allowing them to successfully return it.
"Use a spiderweb to bring the ball in."