Good reply, Rusty (at least, if we are to continue to evade Derek's request!).
The Center Line Model was, indeed, an attempt (among other things) to get away from Final Forms. But it was also, as you have observed, very, very much misunderstood. Among the keys to understanding it better, please consider the assumption that must underlie your statement that "This is why I hesitate to support anything that could be misconstrued as support for a model way to ski." Both my descriptions above, and the Center Line concept as well, portray "the Center Line" as anything BUT "a model way to ski." It is a reference point--no more, no less. You cannot have a "center," after all, without edges! No one would mistake the tone of the tuning fork for music, but most musicians recognize the significance and usefulness of the tone as a reference. Frankly, in my opinion, the Center Line is boring! But it is a crucially significant reference.
To be a valid reference, it must be clearly and universally defined and understood. The tuning fork's "perfection" is not its value as music, but its simplicity and close conformation to the pure "uncolored" universal sound of a sine-wave. And so it is with the "Center Line Reference Standards" (also known as the "Center Line Milestones," among other things). The point was not that they were about "how you should ski," but that they incorporated specific identifiable principles of skill blending, ski performance, intent, and such, "uncolored" by the individual style, mannerisms, and quirks of any particular skier. Those principles can apply at any level of skill--even as the specific characteristics will vary greatly as skills, speed, and confidence increase, and from "real" individual to individual. It was the principles--not the characteristics--the so-called "common threads" of the Center Line--that mattered. And the Center Line represented only one--perhaps arbitrary--point on the spectrum of valid, important skiing movements and skill blends--movements suited to one particular purpose and intent. As such, it was truly no more than a reference line--not an edict.
By contrast, Final Forms were very much about "how you should ski," with every detail and characteristic specified, no distinction made between fundamental principles and "surface" characteristics, and no quarter given to "intent," "purpose," or situational need. Final Forms said "this is how you should ski." Center Line says, "these are the fundamental principles of how you could, theoretically, ski in this specific situation, with this specific intent...if you wanted to." It's a reference--a standardized objective point of comparison for "real" turns.
And we need a reference standard. The immediate predecessor of the Center Line Model was not, actually, the Final Forms days--it was the advent of the Skills Concept(TM). The Skills Concept represented the opposite extreme from Final Forms--it identified not movements at all, but the skills behind the movements. Based on the simple notion that there are really only three things you can do with a ski on your foot--twist or turn it, tip it on edge or flat, and push and pull on it--the Skills Concept described "getting better" as simply improving our skill at managing those three things. Every possible movement and turn type could be explained in terms of how those three skills applied. It was a brilliantly simple concept. But in embracing literally everything you can do on skis, it brought with it the failure to prioritize, the failure to recognize the efficacy of any particular movement pattern in any particular situation. (Of course, good instructors have always been able to incorporate any tool, any model, into their tool bag, using what works, discarding what doesn't, and filling any shortcomings and voids with their own expertise and creativity. No teaching model ever made a good teacher bad!)
The Skills Concept encompassed everything but had no reference line! It brought an era of "hey, any turn is a good turn if you liked it, man..."--fitting for the "groovy" times of the 1970's, perhaps, but hardly contributing clarity or consistency to American ski instructors. It needed references--identifiable "models" and points on the spectrum to which other movements and skill blends could be compared. It needed a road map to help us sort out and navigate the chaotic maze of movement options that the Skills Concept identified. It needed some structure.
And, frankly, we needed something we could hang our hats on--an image that embodied our core technical beliefs about sound, contemporary fundamental ski technique, even as we maintained our stance that intent--not a final form--dictates technique, and that expert skiing required mastery of the entire spectrum of technical and tactical possibilities. We needed to be able to answer the question,"what is a good, basic turn?" We needed something that, without limiting our possibilities, still clearly expressed an American "brand." Why would our customers want to buy our "product" if we couldn't show them what it was, or what was, at least, at its core?
So along came the Center Line Model. Based squarely on the unlimited possibilities of the Skills Concept, it superimposed the missing structure, order, and reference standards--a road map through the chaos. It was (and still is), in my opinion, an absolutely brilliant model. But again, and alas, it was widely misunderstood, raising exactly the concerns that you (Rusty) have pointed out. "Is this a return to Final Forms?" was a common question--you are not alone. Perhaps one problem was that the Model identified only one reference line--for the skill blend of pure, offensive turns. Perhaps it would have been more successful it if also traced the development of movement patterns that fit other purposes--braking movements, for example, and pure carving movements on the other end of the spectrum. The Model did embrace those movements, but perhaps wrongly implied that they were somehow less desirable as "rotary-biased" or "edge-pressure-biased" skill blends.
It wasn't perfect, but the Center Line Model filled a critical hole in PSIA's approach to developing understanding and to teaching the sport. When the Center Line Model was current (and where it was well-understood), American instructors would have had no difficulty in understanding the principles of the "perfect" reference that is Basic Parallel Turn (known as "Open Stance Parallel" in the day). They'd have no trouble--and no qualms about--answering Derek's question.
The lack of replies on this thread speaks to the tragedy of the passing of the Center Line Model--or of some valid replacement for it. It's a simple question--we teach skiing, skiing is about making turns, so what the heck is a good turn anyway? Yes, it is true that a turn's "goodness" depends entirely on the situation and the intent of the skier at any moment--I have long insisted that intent dictates technique--but how can we instructors struggle with defining the principles of "good" movements once that situation and intent are entirely specified? Asking for a clear description of a "perfect turn" should be no more unsettling than asking for a description of effective braking movements, or of an efficient "railroad track" carve. Describing good movements with no outcome specified is, of course, pointless. But when the outcome and all variables are clearly specified, defining principles of efficient "good" movements should be easy for any good instructor.
So Rusty, I understand your reservations completely, I think. But I would ask you to put them aside for a moment and simply answer Derek's question. What is a Basic Parallel Turn? I know that you (and every other instructor here) has an image of it in your mind--and that you actually do use that image as a reference when you watch real skiers. In its "perfect" or "ideal" form, it is not only not "how you should ski, but it is not even how any real, live person could ski, or ever has skied. It is only a mental construct, a simulation, a reference line. But I suspect that you were asked to demonstrate it (among other things) in some way in your exams, the best you could.
So what is it? All objections and reasons for resistance are well-taken. But if that means that we, who teach turns, don't even know what a good one looks like, it does not speak well of our credibility, does it?
Give it shot!