How do propeller planes sync their propellers?
I fly a large 4-engine turboprop.  It uses constant speed hydraulic props, which automatically change blade angle to maintain 1020 rpm.  The sync box (which I was unable to find a picture of) takes inputs from each engine's tachometer generator (where it gets the RPM signal) and a pulse generator, which is a magnetic pickup right behind the prop.  There is a magnet on the number 1 blade of each prop.  With this information, the solid state logic components inside the sync box can slightly change blade angle by sending power to the speed derivative servomotor.  This is different from the answer that @davidinnes provided, but I suspect that he's correct for the type of propellers he's familiar with.  The sync system on the P-3 also takes inputs through a potentiometer attached to the power levers, and can start increasing or decreasing blade angles if you change your power setting quickly.  It works faster than the hydromechanical system and lessens RPM overshoots and undershoots as the prop works to get back to 100% rpm.

In this picture you can see syncrophasing in action.  The props are all slightly out of phase with each other, which reduces noise and vibration. #2 and 4 look pretty close, but it's far from a perfect system.  You can definitely tell the difference from the cockpit when it's working well.  Again as @davidinnes mentioned, you can adjust the system to the change where the point of minimum (or maximum) noise is in the cabin.  We call it &quot;running the buzz&quot; up and down the tube, and it's a great way to annoy a navigator that you don't like