This.
The human body needs a certain amount of oxygen to survive. The only way to reduce the amount of air you need is to reduce the amount of oxygen your body needs. Less movement, more efficient movement, relax and stay calm. Proper weighting means you use less air to maintain your buoyancy. Better trim means less energy to move through the water.
All those methods people use to control their breathing are just gimmicks. The more time you waste counting your breath in & out, the more time you waste that you could be using to actually enjoy your dive.
Yes and no.
Yes the human body needs a certain amount of oxygen to survive.
No, regarding reducing the amount of oxygen your body needs as a way to reduce the amount of air.
The reality is that there is plenty of oxygen in our exhalate. If the amount of O2 was an issue we would not exhale as often and as much as we do. The signal to breath does not come from O2, it comes from CO2. It is the buildup of CO2 in our blood that causes the body/brain to exhale. CO2 is acidic and our body is very sensitive to PH changes/balance in the blood and our tissues. As CO2 levels in the blood rise, the PH level rises as well. This effectively causes an alarm in the brain/nervous system to be set off which triggers the body to exhale. IF CO2 builds, due to breath holding, the nervous system signal becomes stronger. Our breathing function is normally a parasympathetic function basically meaning that we do not need conscious control of it....but we have the ability to have sympathetic control of it and resist the urge to exhale/inhale or voluntarily contract muscles to assist the process.
The above is a bit of an oversimplification so as not go full anatomy/physiology nerd.
The reality though, to improve one's air consumption, one needs to breath less often....to breath less often one needs to reduce their metabolic demand. Increasing metabolic rate increases the amount of CO2 produced by the body causing the body the need to get rid of the CO2.
How to do this (not an inclusive list)
1. Improve fitness level - the better one's cardiovascular fitness is the more efficient the body becomes metabolically. This means that O2 is used more efficiently and less waste product (CO2) for a given metabolic rate.
2. Work on relaxing in the water - stress typically has an effect on increasing metabolism which means higher production of CO2. By allowing yourself to mentally and physically relax in the water your metabolic rate can decrease which equates to less demand for O2 and less production of CO2.
3. Become more efficient - excess and/or inefficient movement increases the metabolic rate which can cause the production of CO2...becoming more efficient in the water by minimizing movement and moving more smoothly and efficiently can reduce metabolic demand and thus reduce CO2 production.
So the bottom line is not about affecting the amount of air we are breathing in but instead affecting how often we need to breath out.
-Z