[thackney]

There is still significant oxygen in the air you would normally exhale. This method would take that quickly to zero percent O2.

For comparison:
http://www.normalbreathing.com/patterns-oxygen-extraction.php

Composition of outer or inhaled air:
O2 - Oxygen 158 mm Hg (20.9%)
CO2 - Carbon dioxide 0.3 mm Hg (0.04%)
H2O - Water 5.7 mm Hg (0.75%)
N2 - Nitrogen 596 mm Hg (78.4%)

Composition of the expired air:
O2 - Oxygen 116 mm Hg (15.3%)
CO2 - Carbon dioxide 32 mm Hg (4.2%)
H2O - Water 47 mm Hg (6.2%)
N2 - Nitrogen 565 mm Hg (74.3%)

[Moltke]

Mouth-to-mouth resuscitation wouldn’t work too well either without sufficient O2 left in the exhaled breath...