Once the system is working, the pressure of the pump lets the water filling in the bladder inside tank.
When the pressure reaches its maximum threshold value, the pump stops. Inside the tank there is the maximum quantity of water. When the membrane expands, it occupies all the volume of the tank. If water is required by the system, it starts flowing out of the tank without using the pump but just using the pressure in the air cushion.
As the process is going on, the membrane deflates until the pressure reaches its minimal threshold value. At this point the membrane is back to its initial dimensions, the pump starts again and a new cycle begins.