A Tennis Ball Pressurizer is a reusable & more advanced version of the can that new balls are bought in, that keeps & restores the pressure inside the balls, by storing them under the pressure. The pressure inside the can, increases the pressure inside the balls, the same way that the pressure decreases when you play with them.
A Tennis Ball Pressurizer is needed to keep the pressure inside tennis & padel balls, since balls are made of porous rubber and leaks air. Tennis and padel balls looses around 20 % of the pressure after one hour of play, and this process then continues for every hour the balls are out in athmospheric pressure. To be able to keep & restore the bounce in tennis & padel balls, they must be stored under pressure, and thats exactly what a tennis ball pressurizer does. (sometimes also called tennis ball saver).