When the engine runs the wire may break and the engine runs on w/o giving any notice of a broken wire. I even have seen engines run OK with most of the wire gone.
Where the wire did break from is pre-detonation. A too hot plug, a too much nitro content or a too high compression will shift the ignition to an early moment, if it is too early there is too much combustion heat damaging the plug and most of the time also the smooth surface of the piston and combustion chamber. The same occurs when the engine is too lean.
Plugs will stay perfect alive at high compressions when the moment of ignition is on the right side.