by 
The Curious Bag
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Vacuum advance is a technique used in most engines to change ignition timing based on power requirements. These systems are often used in addition to mechanical advance, and they help to improve performance, fuel economy and emissions.
Ignition Timing
Since the flame front inside your engine's cylinders travels slower than the piston, the mixture must be ignited slightly before the piston reaches top-dead-center. This lead time is called "timing advance," which changes according to RPM and load requirements.
Timing Advance
As RPM increases, the engine requires more and more advance to ignite the mixture at the right time. Too much advance will cause the engine to work against itself, and too little advance will decrease cylinder pressure past the point of maximum efficiency.
Vacuum Advance
When the throttle is opened fully, intake manifold pressure drops. This drop in pressure signals the vacuum advance to release and allow the mechanical advance mechanism to increase timing.
Location and Identification
Vacuum advance ignitions can be easily identified by looking for the approximately 1 inch diameter diaphragm poking out of the distributor body and the rubber hose that connects it to the intake manifold.
Malfunction
Malfunctioning vacuum advances can lead to power loss, poor fuel economy and excess emissions.
Source:
Resource:
How Vacuum Advance Works (Video)
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