Ball pressure repair can

 A device that restores and maintains the internal pressure of tennis balls to keep them bouncy, preventing them from going flat by creating a pressurized, airtight environment. To use it, you place balls inside, seal it, pressurize to around 14 PSI (like a new can), and let them sit for 24 hours or more, allowing air to re-enter the felt and restore bounce, saving money and reducing waste.