[Excaliber]

On the typical vacuum ADVANCE unit, the more vacuum the more advance. It's straight forward like that, no vacuum, no advance.

However, on certain model cars (pre electronics generally speaking) a vacuum RETARD was also employed. Often it's a 'dual vacuum' diaghram, one side advances and one side retards, each side has a vacuum line attached. Vacuum to either port was controlled by a myriad of mechanical, electrical or othe vacuum operated switches and ports, based on engine load, temperature (ambient, engine, inlet air, etc.). This type of system usually has a nightmare of vacuum hoses associated with it and was largely replaced with the introduction of computer control and EFI.