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.
full, full moon, full phase of the moon, full-of-the-moon - the time when the Moon is fully illuminated; the moon is at the full. lunar month, lunation, synodic month, moon - the period between successive new moons (29.531 days) anomalistic month - period between successive perigees; approximately 27.5546 days.