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.
March: Worm Moon. The Full Moon in March is the Worm Moon. It is also called Lenten Moon, Crow Moon, Crust Moon, Chaste Moon, Sugar Moon, and Sap Moon. more. December: Cold Moon. The December Full Moon is called the Cold Moon, Long Nights Moon, the Moon before Yule, Oak Moon, and Wolf Moon. more