question N2 Sync button

[SenorSloth]

So, i almost always used airbus or GA aircrafts during my simming career and started recently using the CL650

i noticed a N2 Sync button and i would really apreciate if someone could explain what that does and when to use it (or link a video) because i couldn't find anything detailed enough to understand its use.
Thank you all in advance. 

[robder]

I found this discussion interesting:

https://www.airliners.net/forum/viewtopic.php?t=727787

[sztrova]

I read it, but when I push it nothing happens. 

[Graeme_77]

N2 sync is there to force the ATS system to synchronize the engine core speeds, and not the fan (N1) speeds as it does normally.  There isn't a practical use for this in normal operation, but if you select it and walk to the rear cabin you'll hear the distinctive sound of the fans being out of sync with each other. 

You need to be in autopilot ALT hold, with a mach indication on the PFD tape rather than speed and the aircraft on the bug target mach.