Refer to figure.
A magneto is a self-contained, engine-driven electrical generator designed to supply a high voltage to the sparking plugs. This is achieved in a timely sequence for a precise compression stroke.

Each magneto is controlled by its own ignition switch. The switch isolates the magneto’s ignition circuit by connecting the circuit to Earth. In order to completely switch off the ignition system, the primary circuit needs to be grounded. In this condition, it is said to be ‘grounded’. However, if the grounding wire is broken, the magneto will keep working normally and the engine will continue running. Another situation where the engine can keep running is when an excessive carbon deposit has formed in the cylinder head. Because the deposit is still hot enough when you arrive at your parking position after the flight in order to ignite the charge without the spark from the plugs. The effect will be a hardly rumbling engine that is still running.

Note: If the primary circuit is grounded, it will automatically ground the secondary circuit as well.