Steve, The secondary ignition system is basically a spark transmitter with an antenna attached. It is also a resonant circuit (tuned circuit) because of the inductance of the wires inductance and the distributed capacitance of the coil (two wires in parallel is a capacitor). By adding a resistor to an L (inductance) C (capacitance) circuit you reduce the intensity of the oscillation and lower the frequency the circuit resonates.
In layman's term's, you reduce the intensity and move the harmful interference so it won't cause any objectionable operation to other devices.
In layman's term's, you reduce the intensity and move the harmful interference so it won't cause any objectionable operation to other devices.