Each stroke of the piston fills one chamber and empties another, which nearly doubles the flow rate (less the volume of the piston rod) over a single-acting force pump. It also smooths the flow.

From the outside this pump can take many forms, but the basic principal of operation will be identical.

(This is a 32-frame animation, converted to animated GIF format and then super-compressed for Internet transmission. (Note: On some Internet browsers it may be displayed much less smoothly than the CD-ROM version, where the frames are timed to a thousandth of a second.))