What about nose breathing?

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Nose breathing involves a little more resistence to airway flow versus mouth breathing. The resistance also provides some back pressure into the lungs - some PEEP, for those familiar with the discipline. This might explain being able to expel a little more air through the mouth following complete exhalation through the nose. The concepts of PEEP, and natural PEEP are well- studied and used. From "Artificial noses" in ventilator circuits to pursed-lip breathing when you get a runner's cramp. I also doubt the little better alveolar air exchange (they will not collapse) you get from a complete mouth exhalation makes any noticeable physiologic difference.

Several years ago there was a lot of todo over Rhinomanometry testing of nasal airway resistance, used for example, as an assessment in allergy clinics or as a protocol in the work-up for sinus surgery. But, no one has really demonstrated that testing results in anything uniquely clinically meaningful. There are gadgets for Rhinomanometry testing (and even an AMA CPT code), and also some for Acoustic Rhinometry for measuring nasal obstruction around so I'm sure there are practitioners of one sort or another advocating their use for one reason or another.

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