Timing by Vacuum Reading?

From Super Chevy, May 2016. Yes, it is a Chebby but believe it or not, the same principles are involved. Only the numbers will change, very slightly.
p. 56 "Plugging your vac adv unit into a direct [ manifold ] source will allow it to engage at idle, which is good for a number of reasons."

p. 58: " Plug the vac adv into a ported source & check idle rpm. Now switch to a direct source & check idle rpm again. We'll bet money the rpm increased. Why? Because the additional ign timing....allowed the engine to more effectively burn the A/F mixture. It therefore produces more power [ even at idle ] and rpm rises as a result".