When an engine is at idle, it experiences high manifold vacuum. If you have ever popped a vacuum line off of an idling automobile engine, then you have heard the loud hiss that results. That is manifold vacuum suction. Because the fuel is traveling so slowly and the engine is under no load, the vacuum in the engine operates the advance on the distributor, which then advances the timing so the engine idles smoothly and the spark is happening at the correct time. When the accelerator is depressed hard or when the vehicle is under a good load, there is essentially zero manifold vacuum. Therefore, the advance mechanism on the distributor is not actuated at all, resulting in no timing advance.
Vehicles equipped with vacuum advanced distributors (usually older models) have a "pod" on the side of the distributor, with a rubber hose attached to it. The hose connects directly to the intake manifold of the engine so that it receives the full vacuum, or lack thereof. At idle, when the manifold vacuum is high, the tube attached to the advance pod will suck in, causing the distributor to advance the timing. There are springs inside the distributor which hold the timing mechanism in place. They remain in that position until the manifold vacuum affects them. Newer automobiles sometimes use a centrifugal system that uses weights instead of vacuum. The problem with them is that they are based only on engine RPM (revolutions per minute) and cannot detect whether or not the engine is under a load or not. Because of that, the vacuum advanced systems are much more efficient at advancing or retarding the timing to the engine's needs.
A vehicle's base timing point (for instance, if the manufacturer calls for 10 degrees after top dead center) is known as static timing. When an engine is idling and the vacuum is high, it might advance the timing as much as 15 or 20 degrees, which places the engine at an actual, or "total," 25- or 30-degree spark advance. Under acceleration, when the manifold pressure is low, the timing returns to normal. Once the vehicle is at cruising speed and the pedal is steady, the vacuum returns to a higher state because a large spark advance is no longer necessary. Employing the use of electronic timing lights can really make the job easy, but they aren't cheap--a decent model will cost around $60.