You install them in the outlet hose of any diaphragm pump with a pressure switch. They let you restrict the flow of the pump without damaging the pressure switch or relay by excessive cycling. In other words it helps not burn up pressure switches in 12 volt Fatboys!
How does it work?
When the pump starts, liquid enters the reservoir compressing the bladder until the maximum pressure for the pressure switch is reached then the pump shuts off. When water is demanded, pressure in the air chamber forces liquid into the system. Pump stays off until the minimum pressure is reached on the switch. Then pump turns on, reducing cycling leading to longer switch and pump life.