A spring-loaded coupling between a magneto and an aircraft reciprocating engine that causes the magneto to produce a hot and late spark for starting the engine. The rotating magnet in a magneto must be turned at a fast rate to produce a hot spark, but when the engine is being started, the magneto turns slowly.

Arms inside the impulse coupling contact stop pins in the magneto housing and hold the magnet while the crankshaft turns. A heavy, flat spring winds up as the crankshaft continues to turn, with the magnet held stationary. When the crankshaft turns about thirty degrees past the point at which ignition normally occurs, a cam forces the arms off the stop pins, and the spring spins the magnet at a fast rate.

When the magnet spins, it produces a hot spark. And since the spark occurs late, the piston is moving downward, preventing the engine kicking back. As soon as the engine is running normally, counterweights on the arms sling outward and hold the arms in, away from the stop pins. This keeps the impulse coupling disengaged so it has no effect on the ignition timing for normal engine operation.