Pump operator is the skill related to the pump operating labor. Pump operators use screw pumps to pump liquids manually. In order to use the pump, simply query the pump, and hit Enter to "Start pump manually". When you want the pumping stopped, simply repeat the process.
This is useful for starting a perpetual motion power plant, or to move liquid on short notice and with little investment in machinery. However, it's not a good idea to put fortress-critical pumps on manual labor, as pump operation requires continuous dwarven labor.