Final answer:
To turn off a magneto, the primary circuit must be grounded or short-circuited, which stops the current flow necessary for ignition.
Step-by-step explanation:
The question pertains to the function of a magneto, which is a type of ignition system used in some engines, rather than a transformer circuit, which was the subject of the provided reference material. Nevertheless, to turn off a magneto and therefore stop the engine, typically the primary circuit must be grounded or short-circuited. This stops the current flow through the primary winding, preventing it from producing the necessary magnetic flux changes that induce a secondary current, which otherwise would generate the spark for ignition. In many magneto systems, this is accomplished by moving the ignition switch to the 'off' position, which typically grounds the primary circuit.