In the practice of slow breathing make sure that the lungs are as nearly emptied as possible on the expulsion of the breath before beginning a new inspiration, as this gives extra impetus to the fresh supply of air and strengthens all the breathing muscles.
If this is not done, moreover, the effect is like two people trying to get in and out of the same narrow door at the same time.

– the great Enrico Caruso from “the Art of Singing”