Capacitance, inductance, black magic mumbo jumbo... what ever, shielding the plug wire reduces the spark quality. It is easily adjusted for with a battery powered system, less so with a mag not designed for a shielded lead. Even bumping up the power to get the needed spark the radiated RFI is reduced. About 80% of the radiated noise is rom the spark plug itsself and shielding the plug is often all that is needed.