This is only because you're not able to recognize your urge to breathe for what it is...a buildup of CO2 rather than a need for O2. With practice in breathing exercises, you learn to recognize the start of contractions (in the diaphragm) as a stage in the breath hold that's typically around 1/3 of your maximum. With more practice, you can start to recognize more subtle symptoms of low O2...tingling in the extremities is the easiest to notice, but there's a few more.
Hyperventilation is stigmatized in the apnea community because it doesn't increase the level of O2 in the blood. This can be measured with an oximeter...you'll never get above the 94%-98% saturation you achieve at rest while breathing comfortably. All hyperventilation does is purge CO2. During a breath hold, CO2 levels rise and O2 levels fall. The body detects the CO2 rise easily since the PH level becomes more acidic (carbonic acid). But if you've purged your CO2 from your body, it's entirely possible to run out of O2 without the buildup of CO2 being great enough for the body to register the change in acidity and trigger the urge to breathe. And running out of O2 underwater leads to black outs, especially in shallow water as water pressure drops fastest when surfacing those last 10m-20m.
Source: I've practiced apnea/freediving for the past ~7 months and my personal best is 6:48. My training plan is to hit 8 minutes on my 1-year anniversary of beginning my training. Hyperventilation is counterproductive for me since I need to be as relaxed as possible during my holds. My current breathe-up is a 2-1 short to long breath cycle for about a minute then a 3/4 breath, full exhale and a full breath in. This is a slight over-breathe, but not enough to purge much CO2.
Maaaats' answer is excellent, but I thought I'd add one bit for non divers.
When you learn to dive, the most fundamental fact you learn is that the every 10.5m you descend under water adds 1 atmosphere (bar) of pressure. It's customary to round that down to 10m to make the calculations both easier and more conservative. But the important realization from this calculation is that pressure changes more rapidly near the surface. Descending those first 10m doubles the pressure. From 10m to 20m only adds 50%. From 20m to 30m adds even less, 33%.
And it's that percentage change that is most noticeable underwater. You have to equalize the pressures in your ears and sinuses most in the first 10m under water. And since pressure effects your body's ability to deliver oxygen to the brain, that rapid decrease in pressure near the surface when surfacing is where someone low on oxygen will pass out.
This also explains another part of maaaats' answer. Since a freediver is wearing a wetsuit that compresses with pressure (and the lungs also compresses with pressure), their buoyancy will depend on depth. Being neutrally buoyant at 10m deep is a common practice for all but the most experienced freedivers. This means for those first 10m, you actually have to kick your fins to go down or else you'll just float back up to the surface. But beyond 10m, you can just let gravity take you down. And it works the same way coming up. You actually have to kick to get back to 10m, but once you pass that point, the buoyant force starts to help you get to the surface. Experienced freedivers might actually exert themselves less in those last 10m since they've got that assist from buoyancy.
> it's that percentage change that is most noticeable underwater
Ahh! I figured I was missing something, because the pressure change is linear, but yeah... the percentage change is different. Interesting, and thanks for explaining.
When I freedive, I'm often weighted so that I'm neutral buoyant at -10m. So the last 10 meters I will start to float upwards, and thus go faster.
But that's not the issue here. It's the pressure difference when ascending.
> [1] Ascent-induced hypoxia is caused by a drop in ppO2 as ambient pressure is reduced on ascent. The ppO2 at depth, under pressure, may be sufficient to maintain consciousness but only at that depth and not at the reduced pressures in the shallower waters above or at the surface
