Force Pump (LSE) Demonstrate how a pump works! Our pump uses atmospheric pressure to move liquids. When you raise the piston, you decrease the air pressure below it and cause a partial vacuum. The push of the atmosphere on the water in the lower basin forces some of the water up through the lower valve. When you then lower the piston, output valve opens and the water trapped in the cyllinder is forced through the spout. Color may vary. Qty :