Manifold Vacuum Experiment

The vacuum advance canister diaphragm acts against a spring. Adjusting the canister increases or decreases spring tension so the diaphragm moves a different distance for a given amount of vacuum being applied. When vacuum is released it only takes as long as the spring can push the diaphragm back which is essentially instant.

If the engine is pinging under light load because of too much vacuum advance, the spring is adjusted to increase pressure against the vacuum canister diaphragm so there is less ignition advance at cruising engine vacuum.