A diaphragm switch is a pressure-activated switch that turns the pump on and off. The diaphragm is a flexible membrane that moves up and down in response to changes in water pressure. When the water level in the sump pit rises, it pushes against the diaphragm and activates the switch. This turns the pump on, and it begins to remove water from the pit. When the water level drops, the diaphragm relaxes and deactivates the switch, shutting off the pump.