By letting his chest relaxation flow down to the sound source, the music player secures the continuity of the internal propagation from inhaling to exhaling ; such a configuration then brings a strong support to the vibration carried by the exhaling, from the very end of inhaling.
Exhalation is primarily a passive activity during non-exerted breathing, since the diaphragm relaxes while gravitational and resetting forces of the chest and the lungs act as a spring during exhalation, which narrows the chest.
Exhalation must perform slowly and regularly in order to play a quiet melody.
If the chest is expanded, as with inhalation, exhalation automatically begins when the muscles relax and the air is exhaled without any significant muscular contraction.
This flexible system of controlled breathing is typically called breath support. The amount of emitted air is therefore controlled by a flexible coordination of simultaneously activated inhalation and exhalation muscles.
Claudia Spahn, Bernhard Richter, Johannes Pöppe et Matthias Echternach,
Physiological Insights for Players of Wind Instruments (DVD)
(excerpts selected by Guy Robert)