I've talked about this from a different angle in the past, but I feel that I need to repeat something I said before, with a slightly different slant.
I have always modeled safety as a "cone" in the water that is point down. Think of yourself as safe inside the cone, there you're OK, when you start to drift outside of the cone, you are in trouble and when you are completely outside of the cone ... well, sorry chances are that you're not going to make it.
Skill, as created through training, through experience, and (most importantly) as an interactive term combining both, teaches you two things: how to stay, inside the cone (diving within the limit of your training), but also how to feel when you're a little off center and need to move back. Even as you slowly extend your diving to things that, perhaps, you've not had specific training for, you know how to keep yourself in the center of the cone. Even as you go deeper (figuratively) and the circle scribed by the intersection of the plane of your depth and the cone narrows, you know how to stay close to that middle. You know how far off the middle you are. You know how to get back to the middle. Panic is the result of the almost chaotic jump from inside the cone to somewhere outside the cone, with little or no frame of reference as to how to get back. This is sometimes the result of a single, big, possibly predictable or possibly stochastic event, but more often it is the result of a series of small displacements, each oft unnoticed by itself, each multiplying rather than adding to the one(s) that came before. That is what training and experience teaches you to recognize ... the displacements, before one occurs that results in the aforementioned chaotic jump that results in panic, injury and even death.
What is the difference between say Parker Turner struggling to get to the opening till he passes out and the new diver clawing his way to an embolism because his valve was not open all the way? Are either case just bad luck? Are either case the result of not respecting the limits of their training? I don't really know ... some would say that it is a lack of imagination, while others would say that it is strength of will. If I had to guess I say that Parker's lack of panic comes from the habituation of good habits that in thousands of previous displacements from the center were honed to place him right back at the center of his cone, almost without concern as to where he had been displaced to. His experience was that by thinking slow, taking a deep breath and remembering that even if all his gear failed he had four minutes or so solve his issues. Regretfully this missed being enough by a whisker. On the other hand, the new diver imagined above got into trouble within the limits of his training because his training never provided him (or her) with the skills and confidence required to create a self-righting system.
There are some major trespasses that should never be done. Overhead environs are clearly in this category. For the uninitiated a very small drift off center is massively amplified by the lack of a save refuge immediately above and it is this lack of understanding of the amplification factor that creates the danger, what training does is reduce the level of such amplification.
But the blanket, "never dive beyond your training" is patently foolish when we are talking about, say, a depth extension of ten feet. If divers recognize that ten feet creates an amplification factor of "x," but that 20 feet is not two "x," but rather "x" squared, then they will slowly, methodically and with minimal risk extend their abilities to meet the new challenges they face.
But we don't take the time and energy to teach this concept, or more importantly teach students how to use this concept as part of "self-learning," instead the industry takes the easy way out and issues the rather too general admonition: "Never dive outside the limits of your training." That's right up there with the lack or reality of "just say no" and "abstinence training."
