Prior to startup, the pump should be filled with water. Once started, the rotating impeller drives the water inside the pump to spin at high speed. The water undergoes centrifugal force, being ejected outward and forced into the discharge pipe. After the water is ejected, the pressure near the impeller decreases, creating a low-pressure area near the shaft. The pressure here is significantly lower than atmospheric pressure, allowing water from outside to be pushed open by atmospheric pressure through the bottom valve and into the pump via the inlet pipe.