Synch
YSW,back in the days of Flight Engineers,it was done by looking out of the windows at the props, having been set to the appropriate RPM, and lining up the inboard and outboard discs.If there was a residual `strobe` from them, then a little tweak on one rpm lever would adjust the strobe effect to zero.Usually each rpm gauge had a small `synchro-wheel` on it which was also used to synch left/ right sides. Then it would be finely adjusted by the FE to get rid of any particular `residual beat`/drone through the airframe.
Nowadays , the systems are all electronic , using either *2,or*3 engine as a Master engine, and each engine has an electronic actuator on the governor assembly, going thru` a phase and speed sensor unit ( phase unit takes the position of the master blade on each prop). The result is the governor unit on each prop is `nudged` to adjust blade pitch and hence Rpm, until they agree with the Master engine- ie synchronised, then adjusted as necessary to `synchrophase` each prop to reduce residual `beat` or vibration/noise. It really makes a significant difference to noise and crew/pax comfort to have them well set-up....certainly in an Albert !!