To understand the function in some detail lets consider one stroke. During the backward motion of the plunger, the rotation uncovers a fill port (to the right in the figure, just below the magnet valve (solenoid)), and the plunger barrel is loaded with fuel. At bottom dead center the fill port is closed. On the forward pressure stroke fuel is pressurized (to over 120 bar). At this time the Plunger barrel is connected to a particular delivery valve through the channel in the center of the plunger, and a port in the side. When pressure builds up to the delivery valve opening pressure, the valve will open and deliver high pressure fuel to the injector.

When the desired amount of fuel has been injected the spill port opens (located under the control sleeve in the figure), and the pressure quickly drops. This causes the delivery valve to close. During the rest of the stroke fuel is "spilled" through the spill port instead of being injected into the cylinder.

The position of the control sleeve controls at what angle the spill port opens, and thus determines the amount of fuel injected, in other words it controls the metering. The control sleeve is moved in response to a combination of accelerator position and engine speed. The latter is determined by a mechanical governor.