Ginko Biloba and an aspirin are also a good safeguard on high altitude flights even ones at levels where you do not quite need oxygen.
Pressure breathing as described was developed in WW2 AND DOES WORK.
To the system of ridding CO2 allowing a higher concentration of 02 i would add with pressure breathing on inhalation to full lung capacity tighten stomach muscles and the diaphram effectively compressing the full lung of air for a few seconds.
Pace