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.
Hereistheformula: New  Moon  is  a  time  for  activation,  new  beginnings,  and  growth;  a  time  to  initiate  projects.  The  Full  Moon  is  a  time  for  harvest,  and  as  the  moon  wanes,  it  is  best  to  focus  on  wrapping  up  projects  and/or  completion.