Thereu2019s a flexible gastight membrane inside with a gas charge on one side, the water goes into the other side, the membrane splits the tank into two compartments. As the tank pressurises the gas compresses and the membrane stretches to allow the tank to fill with water until the compression limit is reached. When the pump switches off the compressed gas (usually air) keeps the pressure in the tank high. As water is discharged the pressure reduces until a pressure sensor starts the pump again and restarts the cycle.This arrangement is used to prevent the pump from short-cycling and to even out the pressure in the fed main.