The membranes are connected by a central shaft, and create a separation between the compressed air and the fluid. A centrally located air valve alternately pushes one of the membranes to the side. As a result, one membrane expels fluid, while membrane number two draws new fluid in. The suction is the result of a partial vacuum that is created when the air behind the membrane is pushed into the atmosphere; when the vacuum occurs, atmospheric pressure pushes the liquid into the empty room. In the next stroke, the process is reversed: one membrane pushes liquid out of the filled chamber, while the empty chamber is filled with the help of atmospheric pressure.