It runs the petrol mixture lean at part load which is an economy tactic which works only if ignition is advanced, as the rate of combustion is much slower in these circumstances. At part throttle, many depressions (or ‘vacuum’) are a good guide to engine load, and is thereby, used as a indicator to advance the spark timing.
It increases the engine life by more complete combustion and thus, leaving less unburned fuel to be washed away by the cylinder wall lubrication (also called, piston ring wear), and less lubricating oil dilution (for example, camshaft, bearings, etc.). Vacuum advance works by using various vacuum sources to advance the timing at low to mid engine load conditions by rotating the position sensors (like, reluctor stator, contact points, optical sensor or hall effect, etc.) which mount the plate in the distributor in respect with the distributor shaft. Vacuum advance is reduced at wide open throttle (WOT), causing the timing advance to come back to the base advance in addition to the mechanical advance.
One of the sources for vacuum advance is a small opening located inside the wall of the throttle body or carburetor, just beside to but slightly upstream on the edge of the throttle plate. It is termed a ported vacuum. Because of the opening’s presence here, there is very little or no vacuum at idle, hence negligible advance. Other vehicles use vacuum directly from the various intakes. This allows full engine vacuum (and therefore, full vacuum advance) at idle. Some of these vacuum advance units have two vacuum connections, both at each side of the actuator membrane, connected to both various vacuum and ported vacuum. These units will both advance or retard the ignition timing.