I agree with CGeib and Epic and others who prefer the expression "flexion-extension" over "absorption-extension," most of the time. My concern is not so much with the term "absorption," though, even though it clearly does have different definitions that can lead to confusion itself. My concern is that "absorption-extension," which would appear to imply alternate movements (first you do one, then you do the other, right?), actually mixes causes and effects. In other words, "extension" describes a movement--a cause, while "absorption" describes an outcome--an effect. That is what is most potentially confusing about the expression.
The movement cycle is much more accurately described as flexion-extension. Flexion and extension are both movements, and one is the reverse of the other, as the expression suggests. Combined, they can accomplish many things--one of which is "absorption." Yes, "flexion" generally accomplishes a reduction in the pressure created when you hit a bump, so it is closely tied to the outcome of "absorption" of that pressure. But again, one (flexion) is a movement, the other (absorption) is an outcome.
Furthermore, while we generally "flex to absorb" the impact of a bump, flexion does not, necessarily, reduce pressure--nor does extension always increase it. More on this in a moment....
In the process of absorbing the pressure changes in a mogul field, we may well flex low even if there is no bump to absorb. Often, we'll flex low simply in anticipation of the need to extend later. If there is big "hole" coming up, for example, we need to flex low before it, just to prepare for the vigorous extension of our legs down into the hole. It's all part of the overall picture of "absorption" in bumps.
So both flexion and extension are involved in the process of absorption in bumps. And out of bumps as well. Flexion and extension are the tools of pressure management in general. Whether the need is to smooth out the bumps like shock absorbers, or to enhance pressure differences to bend the skis for carving, or to launch ourselves off a jump (and absorb the landing--with flexion), flexion and extension are the movements we use. Absorption is one possible result.
Absorption, as part of the spectrum of pressure management, should not be considered an "on-off" thing. It's not something you either do or do not do, but something we modulate, apply as needed, and as much, or as little, as needed. Generally, "absorption" in this respect implies some degree of reduction in the pressure or force applied to the skis (with the curious exception that The Engineer raised in the first post, where "absorption" can describe "pressuring against the bump to let gravity slow you down").
And in this sense of absorption implying a reduction in pressure (smoothing the impact), it's important to note that flexion does not necessarily accomplish it. It's not so simple that flexing (shortening the legs) reduces pressure and extending the legs increases it. Indeed, the change or pressure actually has nothing to do with the direction of the movement (flexiing down, or extending up). It has to do with the direction of the acceleration, not the direction of movement. Pressure is reduced when the body accelerates "down" toward the ground (or when the feet accelerate up, pulling away from the ground). And vice-versa--pressure increases when the body accelerates up, caused by an upward push from the ground. But it is possible to accelerate up while moving down (flexing). It's what happens when the downward movement slows, as when you near the bottom of your flexion movement. As anyone who has ever experimented with up and down movements on a bathroom scale can confirm, if you start tall and then drop (flex) suddenly, the pressure (weight) first decreases, and then increases beyond your original weight as you slow the flexion near the bottom. Likewise, you can accelerate "down" while moving "up" (extending)--which describes slowing the up movement.
Again, absorption (reducing pressure) describes acceleration of the body toward the feet--either a speeding up of the down (flexing) movement, or a slowing down of the up (extension) movement. And vice-versa for adding pressure. (Bumps, with the up-down movement of the ground, add another layer of complexity, where we must consider the accelerations not just of the body "up and down" but really of the feet and center of mass toward and away from each other.) But clearly, equating "absorption" (reducing pressure definition) with "flexion" is itself confusing--because it is not necessarily accurate.
In this light, I agree with CGeib that it might be worthwhile to explore how different muscle contractions influence the outcome of flexion and extension movements with respect to absorption. Concentric and eccentric contraction are the keys to "speeding up the down movement" and such....
Furthering the potential for confusion, there is yet another definition of "absorption" that has come up in these discussions that has not yet been raised in this thread. All of the definitions raised in this thread so far have described absorption as an outcome managing pressure (or force). Absorption can also be used to describe energy management (which is not the same as force management). When friction, for example, slows us down, it involves converting the energy (kinetic energy) of our speed to a different form of energy (heat and sound), or transferring our kinetic energy to a different body or object (the snow, or the air). As Jamt has suggested, a less confusing term here in a skiing discussion might be "bleeding," as in "bleeding off some energy," simply to avoid using the term "absorbing" with a completely different meaning from the more traditional (in skiing discussions) definition of "reducing pressure."
Like most terms, "absorption" is not itself confusing. Nor is it enlightening, though, without further explanation. Weems once observed that "language is a terrible way to communicate." Rarely can we rely on a term to do our explaining. Almost always, we must explain our terminology. Jargon is only useful if everyone in the conversation uses a term the same way--which is never a known fact until it is discussed in the first place.
Discussions like this one are highly useful, but not for defining terminology. Their use lies in exposing and exploring the issues behind the terminology. And most importantly, their usefulness lies in recognizing that the terminology itself is not clear. It does not communicate a clear, unambiguous idea. We cannot change that. Even if we were all to agree on the "correct" definition and use of a term, it would not help in the next discussion with someone else. But the awareness that the term is ambiguous certainly can help--if it encourages the speaker to explain himself, to define his term, and to recognize that others may well use it differently.