Lister,
The impulse coupling is a spring loaded mechanical coupling which delays rotation of the magneto during an engine start. After a short period of rotation the device allows the magneto to catch up...in effect spinning the mag shaft faster than the engine is turning it. The faster it turns, the hotter the spark and the bigger the voltage impulse...making for easier starting. After the engine is running rotational forces move the spring loaded arms, which make the impulse coupling work at low speeds, out of the way. The impulse coupling only works at very low engine speeds.
When you (carefully!) pull the propeller through in the normal direction of rotation, by hand, the mechanical click you hear is the impulse coupling winding up and then releasing.