Ok you read the title, How do they work.

I would think when I have both engines running the exact (or Close) RPM the needle would be in the middle.

I always try to run them at the same RPM while crusing and just notice the gauge is no where close to the middle.

i have also wondered. good question!

Anyone? I am guessing off the tach?

Come on there are plenty of people smarter than me out there

i believe the impulse off the neg coil lead fires a coil inside the scope[ same as a tach} but in a sync. scope there is a coil for each engine. when the potential across each field is 0 the needle should be mid scale[needle straight up] as on engine spins faster/slower the field becomes stronger/weaker and one coil overpowers the otherand drives the indicating needle accordingly.

You know you are synced when your motors make kind of an occilation sound. You could have a bad tach or a bad sync guage if they seem to be off.