A Kompensatorzapfhahn is a beer tap that allows for adjusting the pressure drop that happens inside the tap. Normally draft systems need to be statically balanced such that the pressure drop inside the beer line and the tap match the serving pressure. If that is not the case a sudden pressure drop may happen at the tap, the CO2 bound to the beer will suddenly be released and the beer foams up. With a Kompensatorzapfhahn (compensator tap) a build in pressure drop can be adjusted to match the needed pressure drop thus removing the need to balance the draft system through the length and diameter of the beer line.
This done though a stainless steel cone that can be pushed into a cone shaped opening through a lever movable from the outside (Figure 2). The further this cone is pushed in, the narrower will be the gap that the beer has to flow through. As a result the higher the pressure drop will be over that distance. Because the pressure drop is not sudden but gradual over a longer distance the CO2 will not suddenly escape from the beer. After that pressure drop there is a valve that allows the beer to flow through the tap into the glass. But unfortunately a large pressure drop, as it is necessary for short beer lines found in a Keg-o-rator require a fairly strong restriction of the flow rate which results in a slower overall beer flow rate. Nonetheless the adjustable flow rate also allows for serving beer at different pressures without having to design the draft system for a compromise between different serving pressures.
Another interesting feature is the "Schaumtaste" (foam button). It is not actually a button on the tap by a feature that is activated when the tap handle is presses backward. In that position the beer is forced though a small opening which causes it to foam up and the foam is directed into the glass. While this feature is definitely not a necessity and a good Wirt (host) should be able to produce a nice head on a beer without this feature it is definitely a nifty invention. Compared tho the foam that is produced while dispensing beer in the normal mode, this foam is much creamier and comprised of much smaller bubbles.
These taps are widely used in Germany but largely unknown in America. So far I have found 2 places in the US that sell these: