Tennis balls are made with an internal pressure of 14 PSI, and packaged in a pressurized can. Pressure is released when you open the can and pressure in the ball escapes through it's porous rubber. Hitting the ball during play forces the internal pressure to escape even more. Balls lose new bounce quickly after you open the can.
Tennis Ball Saver surrounds the balls with 14 PSI pressure during storage. This stops the natural leakage from balls. Also if stored for about 4-8 weeks, it will force pressure back into the balls giving them new ball bounce. To test balls, drop them from 100" To be good they should bounce over 53".
Don't throw good balls away for lack of bounce. Keep them fresh in a Tennis Ball Saver!