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 first phase is the New Moon at which time no light is emanating from its surface. Beginning at zero degrees it will have completed its cycle at three hundred and sixty degrees. One way to describe these phases is to use the analogy of the plant. At the New Moon the seed is in the ground preparing for germination. It is implanted in a new beginning. In the early degrees of the Crescent Phase, the sprout is drawn up from the earth by the initial impulses of light.