You increase the oxygen scores, alveolar PO2 from 120 to 130 mmHg on the first few deep breaths while actually having minimal effect on CO2 elimination.
Yes, I agree, these numbers may sound very impressive - The difference between 120 and 130 is roughly
8%! Who would not want improving his/her PB by 8%!.
The values are misleading, though. The normal atmospheric pressure at sea level is 760 mmHg (the normal O2 partial pressure in dry air is ~21% which equals to the partial pressure of ~160 mmHg, much less in alveoli). So by increasing the alveolar pressure from 120 mmHg to 130 mmHg, it makes 10 mmHg difference, which then represents 1.3%, and is already not as impressive as the 8%.
This 1.3% represents roughly 100 ml of oxygen at lungs of 8 l of total capacity. The nominal resting O2 consumption is 250 ml/min, and only part of the oxygen available in lungs can be used, let's assume it is 50%. That would mean we can use 50 extra ml of O2 which could increase the apnea by 12 seconds. And I am not even sure that the alveolar oxygen saturation (exhale air) can drop as deep as 50%. I think values around 70% - 80% of the original level are more realistic. That vould mean gain of some 5-7 s.
Now, 5 to 12s, even if not a lot, may still sound interesting enough anyway for most freedivers. The problem is that the extra 100 ml of oxygen can come in thanks to washing out 100 ml of CO2. This assumes normal tidal breath-up without any hyperventialtion or deep breathing, and only one single purge breath to achieve the PAO2 of 130 mmHg. In case of multiple purges, or even a moderate hyperventilation, the total volume of removed CO2 will be much higher. The body produces ~200 ml/min of CO2 in rest. It means it will take 30s of apnea to compensate the loss of the single purge breath (considerably more for multiple purges, or a hypocapnic breath-up). And since low CO2 influences the resistance to hypoxemia (P. Lindholm 2002), and brings other disatvantages, the real gain will be likely much lower (if any at all).
Of course, there are many more factors than just those partial pressures and gas levels. We already spoke about most of them here in this thread. As Eric correctly pointed out, there are much more reserves for stocking oxygen in the venous blood. On the other hand, it already requires rather important hyperventilation, that then brings all its disadvantages with it (hypocapnia, shift of Bohr effect, high heart-rate, cerebral hypoxemia, destabilizing neuronal fibers and consequent muscle tension, depletion of myoglobin stores, arterial dilatation and vasoconstriction to certain degree opposite to the one needed by DR, reduced tolerance to hypoxia, later onset of diving response,...)
All that told, I do not claim that there is no way hyperventilation can be used in freediving for improving performances, or for achieving maximal performances. Perhaps yes, but possibly it is very individual, and it may be very depending on diverse conditions. Perhaps some training adaptation helps, perhaps some methods may work better than others. The main risk is that doing the "correct" hyperventilation may not be easily reproducible, and especially when under stress (i.e. competition, or fighting a fish when spearing) it may very easily turn bad.
It is apparent though that hyperventilation has many more disadvantages and trade-offs for its few advantages, than generally acknowledged. For most freedivers, the safety factor is the only argument against hyperventilation. In fact there are many more performance related disatvantages than just the increased safety risk.
