What method of breathing provides a sudden surge of oxygen into the lungs filling it as full as possible in volume and density. Deep breaths? Shallow breaths? Breaths taken while bent over? Deep inhales followed by further inhales. Breaths held? Movement while breaths are held?
You cannot change the oxygen concentration of inhaled air in the absence of an external source of oxygen. However, you can use your lungs at their maximum capacity doing this:
Frequency and tidal volume are the two components of minute ventilation (= (frequency/minute) X tidal volume), which is the physiologic value you are working to increase.
Position: The upright position is best suited, since the weight of the lung tissue pulling downwards will facilitate inflation
WARNING: using those techniques unsupervised can cause acute respiratory acidosis, which is unhealthy and puts you in immediate danger.