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.
In this demo, the New Moon occurs at the beginning of Day 1. A New Moon appears completely dark because the part of the Moon that is visible is the Moon's night side. During the next week (Days 1-7), less than 50% of the Moon's visible face is illuminated and we see a Crescent Moon. After one full week (Day 8), exactly 50% of the Moon's visible face is illuminated. We see a First Quarter Moon.