Originally Posted by Ftrooppilot

Above 12,200 m (40,000 ft), 100 percent oxygen must be breathed with additional pressure to achieve adequate oxygenation to prevent hypoxia. This is termed Positive Pressure Breathing (PPB). The level of positive pressure is generated by the O2 device as a function of the altitude level required to maintain PAO2 at the minimal value that will allow a pilot to perform flight-saving procedures within a few minutes. In other words, positive pressure is equal to the difference between the environmental barometric pressure and the needed lung pressure (196 hPa) allowing PAO2 to be kept at 80 hPa.

5.3 Technique for pressure breathing
During PPB, breathing patterns are inverted as inspiration becomes passive and easy and expiration becomes active and difficult. As a result, breathing must be consciously controlled to avoid hyperventilation. Practice is required to become accustomed to this reversed breathing pattern.
The best technique for PPB is as follows:

• Establish mental discipline to control breathing
• When inhaling, maintain a conscious tension of the respiratory muscles (diaphragm and abdominal muscles). Control the expansion of the thorax through muscle tension. As inhalation progresses, steadily decrease muscle tension to allow progressive lung inflation
• Pause when the desired lung inflation has occurred
• When ready to exhale, positively increase muscle tension for a steady, smooth exhalation
• Pause and breathe at a rate slower than normal