Stores balls after a tennis match. When Restore is closed it will compress air to 14 PSI which is equal to the pressure of an unopened can of tennis balls.
Once a can of tennis balls is opened, the balls will become flat in a few weeks even if you have never played with them. Storing balls in Restore stops further pressure loss so you can play few more sets even weeks later.
Twist down top seals and compresses air in. The O-ring traps and compresses air as unit is twisted closed.
You can test that it's holding air pressure by submerging Restore into a sink filled with water. No bubbles means it is holding air pressure.