Hi TDK6--good to "see" you! Hope you've had a great season.
Yes, the answer is in the text you quoted (one of the problems with quoting entire posts), but I think you are right to question it.
In my opinion, the description of wedge christie from the PSIA-West technical manual, while not necessarily inaccurate, can be quite misleading and confusing, in several ways. In particular, it appears to reverse a few cause-effect relationships, which at the least tends to place the focus on "what you should do" in the wrong place. Of course, as Bud points out, brief written descriptions like these paint only a small part of the picture--the rest is the image, demonstration, perhaps video, and explanation of a clinician. All good, IF the clinician's understanding is accurate, and consistent with the understanding of all the other clinicians. (In other words, probably not all good!)
"Turn initiated by opening both skis into a wedge with a slight extension." As worded, this line suggests that "opening both skis into a wedge" and "a slight extension" cause the turn initiation, which is misleading. In fact, the wedge occurs as a result of the turn initiation, which is caused by releasing the edge (letting go of the mountain so that gravity can pull you down the hill) and steering (turning) both legs and skis down the hill (at different rates--also a result, not a cause, of the turn). Releasing the edge and guiding both skis down the hill is what you, and your student, should focus on--not "opening both skis into a wedge." If they turn at different rates, you'll have your wedge. If not, you've progressed to parallel!
And "with a slight extension" is at the very least vague and open to misinterpretation--if not just plain wrong. Extension of what? Since the sentence talks about "both skis," it's reasonable to infer that the extension should involve both legs. Bud attempted to clarify that in the words Rusty quoted, pointing out that "flexion [and extension] in the wedge and wedge christie demos should be a lateral movement of the hips to balance against the slight turning forces generated"--which suggests that the two legs flex and extend unequally--"long leg-short leg"--which does accompany--and if active, cause--lateral movement of the body (cm) over the feet. In other words, when we lean into a turn, our inside leg (left leg in a left turn) tends to become shorter while our right leg tends to extend a bit.
If that (long leg-short leg) is what the author of the tech manual intended by "with a slight extension," it is not obvious as written (although he/she does add clarity under the "Edging" heading, with the words, "simultaneous steering & extension of the uphill leg...."). But even if it is what he or she meant, I believe that the cause-effect relationship is backwards, as far as "perfect" wedge christies go. As we move laterally across our skis and into a turn, our legs tend to flex and extend unequally, but it is only necessary to do it actively (muscularly and intentionally) when we need to force our body to move laterally. As anyone who has ridden a bicycle can attest, when we're balanced and flowing smoothly from turn to turn (like a wedge christie should be) there is no need to "force" that lateral movement with active long leg-short leg movements.
I remember a clinic--actually, it was an Examiner selection, unfortunately--in which the candidate suggested that we "imagine our flexed uphill leg at the beginning of the turn is large hydraulic jack, which extends and forces our body across our feet and down the hill...." Well, if you need to force your body across your skis, that's certainly one way to do it. But if you need to do that to start your next turn, you've already made a big mistake (you've finished your last turn not in "neutral," and somehow stopped the accurate "flow" of your center of mass down the hill) and that "extension" represents only a bandaid fix. You should fix the problem on your next turn, so you won't need that "solution" again!
For me, "flexion-extension" is among the last things I focus on in my wedge christies, and when teaching students at that level. First of all, when I hear the simple word "extend" (or "flex"), it does not trigger an image of "long leg-short leg" lateral movement (or of fore-aft movement, for that matter)--it suggests getting taller or shorter. And I think that there is wide-spread confusion of why, and when, we need to get taller and shorter in turns at all levels. Primarily, we do it to manage the amount of pressure on our skis--to absorb bumps, to regain contact with the snow after flying over a roll, to reduce the pressure at the bottom of turns so that soft snow doesn't disintegrate beneath us, and so on. Expressions like "extend to release" and "flex to release" imply a cause-effect relationship that simply is not there. Releasing is a tipping movement. (Yes, it involves lateral movement of the center of mass over the feet, which we can "force" with long leg-short leg movements, but I've already discussed that, and it is not the same as "getting taller or shorter").
In short, I've long been critical of the common advice to "extend to release" or "flex to release." "Well, I mean, 'extend down the hill, or extend into the turn,'" is the common retort, to which I reply, "why not just 'move down the hill?' Why do I need to 'extend' (or 'flex)' as I do it?" To which I am usually faced with a confused, dazed look, accompanied by "well, that's what you're supposed to do, isn't it?"--which betrays a lack of understanding, "knowledge" without "comprehension." There MUST be a better answer to "why?" than "because that's what it says in the manual"!
Now...another reason to "flex" (get shorter) is to allow angulation movements in the feet/ankles, knees, hips, and spine (you can't tip your skis with all of your joints rigidly extended, beyond how much your entire body tips). So it is normal that we flex some joints somewhat as we increase edge angle in turns (note that I did not say "to" increase edge angle). And as we decrease edge angles to exit the turn and release the edges to begin the new turn, there is no particular need to stay "short" (which requires muscular effort), so unless there is some other need to "flex" (eg. to absorb a bump or a "virtual bump" in high-speed turns), we might as well stand a little taller and relax through that transition and edge release. So it is not uncommon, and not inappropriate, to "rise" through the transition in low-speed wedge christies. It is a luxury we can afford in such low-speed turns. But it certainly is not necessary or required to cause the edge release. We may well extend (rise) as we release, but we certainly do not need to "extend to release."
Furthermore--IF we think we must rise in the transition for whatever reason, timing is critical--and commonly missed. We'll rise as we reduce the edge angle, and the edge release will occur at the top of the rise. In other words, we will rise OUT of the previous turn, NOT into the new turn. If you start extending (rising) as you start the new turn, you'll be way too late--and guaranteed to start your turn with a stem of the uphill ski (since the downhill ski won't release until the top of the extension).
So I find it rarely necessary--or helpful--to even mention flexion-extension in wedge turns or wedge christies. Yes, if a student tends to get bent over and "stuck" on his edges, a simple reminder to "stand up" can help let go and stimulate the release. But it's still not a necessary move, and I'd prefer to keep the focus on "flatten to release," which describes the true cause-effect relationship. Indeed, since high edge angles are not yet involved at this level, there is little need to flex low, beyond just the gentle flexion of a basic athletic stance. Furthermore, if I don't bring it up, I won't have to "unteach" "extend to release" later, when we're skiing bumps or high-performance turns that may well require flexion/retraction through the transition. In my own skiing, and in my teaching, I want to keep extension/flexion ("tall/short") movements entirely separate from tipping, lateral, fore-aft, and rotary movements. I want to be able to do each of these things independently, as needed, without necessarily affecting the others. "Extend to release" ties two of these movement pools together in a way that is unnecessary, and that will become detrimental at some point in the skier's development.
I suggest that other potentially misleading or confusing statements in the PSIA-W tech manual, possibly involving reversal of the cause-effect relationships (or implying causal relationships that don't exist), include "turn initiated by opening both skis into a wedge" (remember--both skis turn in the same direction but at different rates, not in opposite directions, and the unequal rates are the result of other factors, not primary causes of the turn) and "the weight shift should be passive yet simultaneous with the release movement of the down hill ski rather than any active shift" (these things can occur simultaneously only with an active, muscular "weight shift," or if passive, the shift will be smooth and gradual, later in the turn, not "simultaneous" with the edge release). All worthy of further discussion!