Agree completely, cdavis. Only thing I'm unsure about is that you seem to be advocating UNDER ventilating, 'hypoventilation', if I understand correctly.
re OP, here's my understanding:
Hyperventilating literally means 'breathing too much', i.e. deeper or faster (or both) than the body needs at that moment.
Purging is a forceful exhalation and a passive inhalation. The intention is (amoung other things, but the relevant one here is) to clear out the stale air in the lungs so that you go down with a tank full of fresh air.
So purging IS hyperventilation.
But go back to first principles. Why is HV a problem? Because it reduces blood CO2 which, in turn
- reduces the urge to breathe, making blackout easier,
- slows the dive response,
- and decreases pH of blood, adversely affecting O2 carrying capacity of hemoglobin.
- ...did I miss anything gang?
This is the effect of reduced CO2
in the blood.
It takes time for HV to effect the blood because it takes time for your body's blood supply to circulate past the lungs and interact with the gases in the lungs, the composition of which has been altered by HV.
Try this thought experiment:
I use my magic powers to transform the air in your lungs to pure O2, just for the last few minutes of your breathe up. Oh no! Without even knowing it you're offloading WAY more CO2 than normal, effectively hyperventilating. Now imagine I do it just for a second. Less of an effect obviously. Or a thousandth of a second? Hell I could turn that air into laughing gas and if it was only for a thousandth of a second, it probably wouldn't affect you.
Point is that time is a factor.
How many people do 5:10 breathing?
I know divers that do 5:10 breathing for a minute or two before the dive, then claim they can resent the blood CO2 by, "throwing in a tidal breath to stop HV", (I've got that in writing from a qualified freediving teacher). But they won't do purge breaths because, "that's HV". Don't make the mistake of thinking HV = fast breathing.
You can do three purge breaths in maybe 5 seconds, and gas composition in the lungs only changes with the inhalation of the second one. If it takes about a minute for blood to circulate then AT MOST 10% of the blood is exposed to the mildly altered CO2 level in the lungs, and then only for a few seconds.
So yes that's HV but it's a TINY amount.
Whether that effect is offset by the benefits of purging is another question.
Personally I've moved away from purge breaths in favour of relinquishing all conscious control of the breath and focusing instead on relaxation and calming the mind.