LITTLE GIANT Float Switch: Normally Open (Pump Down), 115V AC, 10 A, Piggyback Plug, Sensor, Nylon, brand new in the box.
These switches have a diaphragm encased within the switch's housing that senses pressure changes caused by rising and falling liquid levels within a container. As fluid levels rise, building pressure moves the diaphragm and presses against the internal switch to start the pump. As liquid levels drop, pressure is alleviated from the diaphragm, which deactivates the switch and turns the pump off.