The outer (primary) diaphragm senses carburator vacuum just upstream of the throttle butterflies to advance ignition timing; the inner (secondary) diaphragm senses manifold vacuum. Therefore, when the manifold vacuum is high(eg during deceleration or idling) the secondary diaphragm retards the spark. As soon as the throttle is opened, the primary diaphragm takes control and causes the spark to advance. The purpose of this system is to decrease the emission of unburnt hydrocarbons at low throttle openings.