The resistor is a wire coil the heats up under a 12V load reducing the out-put voltage to 6V, those coils are designed to operate on 6 volts, running them on 12V will produce more spark, for a while, but it will cause them to over-heat and produce intermittant fire, and eventually just quit. Back in the day, when those coils were installed with an external resistor, you would find another wire from the starter solnoid that would sent 12V to the coil to produce more fire just to start the engine, but the engine would run on the 6V thru the resistor after it started. I use the coils that don't require a resistor also, and electronic ignition, if the coil requires an external resistor, it is normaly stated in print on the side of the coil, but that may not be stated on a coil with an internal resistor, but thats something you need to be aware of.