The Syncro Spark part of the ignition is a small circuit board, about 1 inch square..It is wired in series with the ignition circuit..It can be set up for full advance at either 4 or 6 thousand rpm, and either 20 or 28 degrees total advance..The timing is set by placing the Hall sensor at the correct spot relative to top dead center, usually 28 degrees..The first flip of the prop retards the ignition to 0 or 8 degrees, depending on the setup of the Syncro spark module..Advance is automatic after that..The advance happens when the crankshaft starts turning faster, like the tail wagging the dog--more rpm, more advance..It happpens is milliseconds, so there is usually no perceptible delay..If the low speed needle is set too rich, however, the advance happens slower and there will be a slight delay in the transition while the crank speeds up..
With mechanical advance the throttle controls the advance curve, linear from idle to wide open...
The price difference of $12.00 is way less what the Syncro Spark board costs...Currently $39.95 from C&H.......