Vacuum in the manifold drops as engine demand increases. If you use ported vacuum you always have a vacuum signal. If you want to make adjustments it can be done using the advance pot on the distributor when you use ported vacuum.
The decreasing or waning phases are when the moon shrinks from the full moon down to the new moon (darkness). As the moon wanes during the 3rd and 4th quarters, this is a good time to prune plants, as the water table is diminishing and so less sap will flow out of the cut ends.
The Moon goes through eight lunar phases, and changes phase every night. The phases are (in chronological order): 1  New Moon. 2  Waxing Crescent. 3  First Quarter. 4  Waxing Gibbous. 5  Full Moon. 6  Waning Gibbous. 7  Last Quarter. 8  Waning Crescent.