When I was searching different materials about hypoxia and hypercapnia, I also found many articles speaking about deadly teenage choking games. I do not remember doing or seeing anything similar when I was a kid, but according to the sources this type of games is very common worldwide, and a quite frequent reason of children / teenage deaths. Check out for example these links:
Choking game - Wikipedia, the free encyclopedia
Hyperventilation - A Deadly Game
Choking Game Education: Deadly Games Children Play: Awareness, Family Help, Statistics, Victims
or more in
Google
I mention it because the mechanism of the blackout is very similar to the one at apnea - it is also caused by brain hypoxia, although not due to the general blood hypoxemia, but rather due to the hyperventilation and subsequent hypocapnia. So if there are numerous deaths due to hyperventilation games among the teens, it is very likely you can die in similar way when training dry apnea.
So the advices mentioned above are definitely good to follow. Also laying on the back may not be the best position for the case you vomit or "swallow" your tongue in unconsciousness. Half sitting, or laying on your side may be safer. When sitting or standing, putting a lot of pillows around you may be also wise thing to do.
