Plain English Meaning

A switch position that turns off one of the engine's two ignition systems so the pilot can test the other one by itself. With the switch on LEFT, only the left ignition system is firing the spark plugs.

Full Definition

On a piston aircraft ignition switch, LEFT (or L) is the position that energizes only the left magneto, allowing the engine to run on that magneto alone. Pilots select this position during the magneto check on runup to verify the left magneto and its associated set of spark plugs are firing correctly, while the right magneto is temporarily grounded .