As the ignition system is normally arranged to produce sparks before top dead centre there is a risk of the engine kicking back during hand starting, to avoid this problem one of the two magnetos used in a typical aero engine ignition system is fitted with an 'impulse coupling', this spring-loaded device delays the spark until top dead centre and also increases the rotational speed of the magneto to produce a stronger spark. When the engine fires, the impulse coupling no longer operates and the second magneto is switched on.Thom 1988, p. 166.
