Reversing coil polarity makes the current travel through the primary (low voltage) coil winding backwards to its normal direction of flow, which effectively puts the primary in series with the secondary winding when the magnetic impulse generates the high voltage spike to fire the spark plug. The inductive reactance, which is the equivalent to resistance induced when current flows through a coil winding, reduces the available voltage seen at the spark plug. I've seen that indication many times on an oscilloscope when diagnosing poor performance complaints.
On distributorless ignition systems where two spark plugs fire from opposite ends of a single coil secondary winding, half the firing voltages will be noticeably less on a scope pattern when the spark originates at the "cold" ground electrode and jumps to the insulated center part of the plug.
Jerry
