The magneto switch controls the primary magneto circuit by grounding it.
This is done to isolate individual magnetos during the rpm drop check.
If this wire is broken and therefore cannot be earthed - then the
magneto will continue to supply current to the engine and it will continue to
run.
The correct answer as stated is excessive carbon formation, this forms hot
spots which remain heated and continue to ignite the mixture even though the
plugs are not sparking!