Synchrophaser

ALENIA Surveyor

Nothing special about this twin prop regional airliner, unless you notice that the propellers are in phase. The photographer, Wilfried Broemmelmeyer, comments: "Looks like the synchrophaser is working perfect." Synchrophaser? Oh, yeah, I used to have one of those installed my car, right next to the slankerator. 

Okay, slankerator is just something I made up, but a synchrophaser might be a real thing. At least SKYbrary thinks so:

A refinement of the propeller synchroniser is the propeller synchrophaser. Like the synchroniser, the synchrophaser precisely matches the RPM of all of the propellers. However, the synchrophaser also compares the propeller to propeller position of the propeller blades within their individual arcs and then adjusts them to their optimum relative positions. This refined adjustment results in a significant further reduction in propeller noise and vibration as compared to those achieved by the more basic synchronisation system.

Now I'm wondering how the heck you could accomplish this. I can't think of any technique that would work. Must be black magic.