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.
Other variations indicate more signs of full spring, such as Sprouting Grass Moon, Egg Moon, and Fish Moon (common among coastal tribes). May: The Flower Moon. Flowers come into full bloom and corn is ready to plant. Also called the Corn Planting Moon and the Milk Moon.