An Air Force Pararescue Jumper trainee swims to the finish point, Aug. 17, 2011. U.S. Air Force photo by Staff Sgt. Jonathan Snyder

It’s not easy to join Pararescue. Over 80% of the men who try out for the Air Force's special operations search and rescue corps don't make it (women will get their chance soon. More on that below). Depending on how you measure, that might be the highest attrition rate of any job in the military.

Of course, drop out rates for “elite” military training are one of those statistics that tend to get higher the longer ago you made it.

I made it in 1998, which was, indeed, long ago. I graduated from Pararescue’s Indoctrination course — “Indoc” — as part of a class that started with what I remember being told was about 120. Eight weeks later, 14 of us were still standing, including rollbacks from earlier classes.

So what happened to those 106 who melted away? Maybe a few dozen — less than half, for sure — failed out on one athletic measurement or another. Too slow on a run, not enough pull-ups, something like that.

The rest just quit. Man after man, sometimes in waves in the middle of training, sometimes alone in the middle of the night, usually in tears.

How does that happen? Nobody makes you try out for Pararescue. Everyone who does is young, in shape, and signs over their life to Uncle Sam just for the chance. Who would quit?

It’s the question that, in various forms, you get more often than any other as a PJ: What’s so damn hard?

There’s a simple answer: The Pool.

Specifically, the hours every day that classes spend in “water confidence” training, or “Water Con,” a unique form of voluntary suffering that turns a few volunteers into PJs and keeps many more out.

It doesn’t look like much. In fact, it looks like the opposite of what you think Indoc should look like: mud pits, endless push-ups, carrying Zodiac boats on your head. All of that, and so gloriously much more, is part of every day at Indoc. But after a full day of traditional training, your class jumps on a bus and heads to The Pool on what is a very quiet bus ride. There, as instructors scan for errors or signs of fear, you must:

Swim underwater lengths of the pool, with a surface sprint back.

Dive to the bottom of the pool to tie complex knots.

Dive again to strip off basic scuba equipment, surface, then dive once more to re-don it in order.

Bob up and down with your hands and ankles tied in a drill hilariously known as “drown proofing.”

And everybody’s favorite: buddy breathing, where two students share one snorkel while in an underwater wrestling match with an instructor except the students can’t fight back. If you’ve ever seen a recruiting video or TV special on PJ training, inevitably, buddy breathing gets lots of screen time.

The Pool has other events, like treading water and swimming with weights. But, while difficult, those events have a vital difference: You can breathe.

And that’s the secret of The Pool, the reason hundreds of men with dreams of being PJs stand up and quit in the middle of it. It’s uniquely diabolical not because it’s hard, but because it’s all so damn simple.

You just don’t breathe.

Related: A Navy SEAL’s guide to surviving BUD/S »

Not breathing is unique torture. Unlike any other stressor – cold, heat, fatigue, even injury — the hypoxia of not breathing jiu jitsus an athlete’s strengths into weaknesses.

Within seconds of holding your breath, sensors in your neck’s carotid artery note the dropping oxygen levels in the blood heading to your brain. Cells throughout your body, and particularly in your muscles, begin acting anaerobically, eating up sugar without oxygen, producing both lactic acid and pain. The evil irony is that “being in shape” is supposed to inoculate you to this exact physiological spiral. But even if you can do wind sprints and burpees all day burning sugar and oxygen, The Pool short circuits your defenses in about 5 seconds.

As your arms and legs start to go numb, the large autonomous muscles in your diaphragm, which let a normal person breathe 25,000 times per day without ever thinking about it, start to freak out with increasingly hard, uncoordinated contractions. Those spasms create inadvertent gulping noises. PJs call it “guppying.” It feels like your stomach is eating your lungs and you can hear it echo across the pool.

After 20 to 30 seconds — less if you’re swimming or fighting an instructor — your fine-tuned athlete body is choking itself out. All you have left is your mind.

This is the moment that ends most men’s PJ journey. You have a split-second to decide: do you want to be a PJ more than you want to breathe? Because it doesn’t just hurt — it’s terrifying. Instincts locked deep in your brain suddenly rush to the surface and you must. Breathe. Right. Now.

There’s only two ways out: panic and quit. Or stay calm, focus on the mission and keep going. As the stress goes up, you learn to focus on an internal dialogue to keep your heart rate down and anxiety in check. In a word: Relax.

When your fingers go number tying a knot, you can panic. Or can relax. When an instructor pushes you to the bottom of the pool and stands on your head, what can you do? You can quit. Or you can relax.

The Pool’s direct utility is not to prepares students for combat but for Pararescue’s advanced scuba training. But through 60 years of missions, PJs believe their common thread for success is the mental calm and toughness learned in the The Pool at Indoc. And they are fiercely protective of it. When the Air Force began reviewing Pararescue training for inclusion of women, it surveyed teams of combat rescue officers on the value of each piece of Indoc. They rated Water Con as the most important test to predict eventual success as a PJ, ahead of the school’s running, calisthenics and ruck marches.

Are there shortcuts? Surfers, used to being held underwater by waves, and competitive swimmers — particularly water polo players — tend to have a leg up. And women, who will begin arriving at Indoc soon, may adapt well, with less oxygen-hungry muscle mass and more efficient vascular systems.

But the core lesson of The Pool will never change: Stay calm, stay smooth and focus on the task at hand. Which, in the end, is the heart of every PJ mission: No matter the obstacles, the odds or the hardships, you must complete the mission. And never, ever quit.