Final answer:
When an aircraft's ignition is switched OFF, the magnetos are grounded to prevent the engine from starting or running.
Step-by-step explanation:
When the ignition switch of an aircraft is turned to the OFF position, the magneto (mags) are grounded. This means that the electrical circuit that allows them to generate a spark is intentionally disrupted. Magnetos are a self-powered ignition system that enables the engine to run. By grounding the magnetos, the connection to the spark plugs is broken, which stops the engine from running. It is an essential safety feature to ensure that the engine does not inadvertently start or continue to run when a pilot intends to shut it down. Therefore, turning the ignition switch to the OFF position is the standard procedure for stopping an aircraft's engine.