Thanks for the additional info.
So is SWB a separate phenomenon from just holding your breath too long or is is the same? It seems different even though the results are the same.
The term "shallow water blackout" is often used by people outside the freediving world for all hypoxic losses of consciousness that happen to breath-hold divers. In the freediving world, it is a specific form of hypoxic loss of consciousness (HLOC).
Obviously, if you stop breathing, you will eventually pass out. How long that will take depends on many factors, but blood gas analysis of physically fit, female Japanese and Korean Ama freedivers who harvest food from the sea on a daily basis show significant levels of hypoxia in just one minute of breath-hold time. These women don't dive very deep (average of just 20 - 30 feet) and don't hold their breath longer than about 40 seconds max. I'm pulling up these numbers from memory. They may not be entirely accurate but you get the idea: shallow dives and short breath hold times. They have a reportedly good safety record over a 2,000 year history.
Western freedivers tend to push breath-hold times in three ways:
1. Static - holding one's breath without moving. This is often how kids and even Navy SEALs die competing to see who can hold their breath the longest. Compared to a car, they just idled until they ran out of gas.
2. Dynamic - swimming underwater while holding one's breath. This is like driving a car. The faster you swim, the more gas you burn because your muscles will demand oxygen.
3. Depth - with depth you also add the compression of one's air spaces due to Boyle's Law. If you have twice the pressure, you reduce the volume by half. 3 x pressure = 1/3 volume. 4 x pressure = 1/4 volume. When the pressure is reduced and these spaces return to normal volume, the tissues around these spaces like those in the lungs and sinuses will draw blood back into them causing a reduced concentration of oxygen in the blood and increasing the risk that you will run out of gas.
They may also employ hyperventilation which reduces the carbon dioxide level in the blood. CO2 stimulates our urge to breathe. It's like tampering with the gas gauge in the car so the needle shows more gas than is in the tank.
The most common freediving accident was in shallow water. If we look at what used to be the most popular freediving activity for decades, underwater hunting, we can see how all of these things come together in one dive.
A spearfisherman hyperventilates before a dive so he doesn't feel uncomfortable while hunting. He gracefully surface-dives and uses little energy to descend to a deep reef. He stays still, holding his breath longer than he should because he feels no urge to breathe, and locates a fish. He then moves slowly to stalk the fish burning a little more oxygen. He fires the spear and now must fight the fish. Struggling with the shot fish burns even more energy. At this point, he may have spent a couple of minutes underwater and now feels the urge to breathe. He begins to ascend more rapidly bringing a large fish with him. His muscles require more oxygen as his large leg muscles work to swim up with the additional drag and weight of a fish. It may even still be alive and he may be working to control it. As his lungs and sinuses return to their normal volume, what little oxygen he has left is further reduced. He passes out within a few feet of the surface in shallow water. This was the most common type of open water freediving blackout outside of depth competitions.
The most common in pools was people either trying to hold their breath for a long time or when swimming the length of a pool with an inefficient stroke that burned a lot of oxygen. Even professional freedivers blackout when performing static and dynamic apnea in competitions. There are videos on YouTube showing lots of blackouts.
All of these are just due to hypoxia but terms like deep water blackout and shallow water blackout are sometimes used to describe the point of the dive where a blackout occurred, and SWBO has just become a generic term for a freediving blackout in layman's terms.