Plain English Meaning

The switch setting that turns on both ignition systems at the same time, so both spark plugs in every cylinder fire. This is the setting you fly with.

Full Definition

The ignition switch position in which both the left and right magnetos are energized simultaneously, firing both spark plugs in each cylinder. This is the normal operating position for engine start (after priming, where applicable) and all phases of flight. Using BOTH produces more complete combustion, slightly higher power, and smoother engine operation than running on a single magneto.

Context Anchor

Seen on the ignition switch in -engine , especially during engine start, the engine , and normal engine operation.