OK DA WORD
A Syncro Spark is an electronic RETARDING device.
If you set your engine at anything other than full advance while using a Syncro Spark it will NEVER develop max power.
Its a long explanation but it knows RPM from the first turn. At a carrythrough 180 rpm it retards 26 degrees, below 180rpm it is full advanced so that you can set the static timing. The average modeler can prop a Sachs 4.2 at 450rpm. Most of the small weedies don't need the Syncro because they don't have enough compression at cranking speeds. A Syncro will let the engine idle a bit slower, important on a J-3 but not on an Extra, but add no power on a properly set up engine.
The crossover point of needing or not needing a Syncro is around 30-35cc.