In the same pit as the sump pump there is a ball that floats in water attached to a float switch. When there is no water in the pit that ball is in its lowest position and the pump is off because the float switch keeps it off. As water rises in the pit, the ball floats and when it floats to a certain point the float switch turns the pump on. When the water level falls to the point that the ball is no longer floating, the pump turns off. This cycle happens over and over to keep your basement dry.