Some instruction and coaching organizations are not movement-oriented but instead are skill-oriented9. These look at all possible movements, from all joints and in all planes of motion as an evaluation framework, but describe skiing and focus instruction on "skills" like balance, edging, pressure, pivoting8.
While some use both skills and movements3, a few see skiing as "a sport of movement"1 7, as do we, and describe skiing in terms of and focus instruction on the movements a skier makes. The skier below displayed a lot of skill, but executed a series of movements to get from left to right:
The Planes of motion framework looks at the movements that a skier can make, from a physics perspective (i.e. separate movements in a few "planes of movement") which are then recomposed back into instruction, to create results.
The essentials framework is inspired from high level racing and has more in common with racing and high performance skiing. The essentials are a small subset of movements, organized in a hierarchy, starting with the lower body movements tipping, flexing and fore-aft, complemented by the upper body movements counterbalancing and counteraction.
Here's a more detailed comparison, following along the planes of movement.
The Planes of motion framework describes Inclination and Angulation as the lateral movements for both edging and balance. Angulation includes edging as a form of angulation at the ankles1 to edge the skis and angulation at the knees (flexing), hips and spine, to stay in balance. Angulation is described as the result of these smaller "movements", in no particular order.
This focus on upper body movements however, tends to lead to a lot of hip dumping which, even in a refined form, is very pervasive among recreational skiers and beginning racers, as they tend to drag the skis on edge with the upper body.
Edging actions can be achieved by whole body inclination or by angulation. However, it is the fine-tuning movements in the ankle that, while impossible to see, may distinguish between the optimal edge angle and a less efficient edge angle.USSA SkillQuest
In contrast, in the essentials framework, there is no inclination "movement", and two components are clearly identified: tipping is a separate (primary) movement to edge the skis from the ankles and feet, with a special emphasis on tipping the inside ski while counterbalancing is the upper body lateral movement (at the hip/spine) that complements tipping, to stay in balance - continuing a keen focus on separating lower and upper body movements.
In the Planes of motion framework, edging the skis is seen as a result of angulation or inclination. Because tipping is not identified as a separate movement, releasing is seen only as "unweighting" in different ways and reducing inclination, but in reality we also need to flatten the skis, or "untip" them.
In racing, there is a lot of attention given to edging and tipping while angulation is split between a keen focus on moving the lower body and using the upper body for balance only5 etc. Also, the releases start with ankle action4 followed generally by flexing the long leg... in fact, the release is often equated to decreasing the edge angle of the ski3.
In the Planes of motion framework, fore/aft movements are not specified other than being a result of complex movements from all joints, to stay in balance and use the ski.
In the essentials framework, like in high level racing, the ankle dorsiflexion/plantarflexion and moving the skis back and forth5 (together or the free foot only2) are the fore/aft movements, complemented by the upper body only as needed.
In the planes of motion framework1, the rotations of the upper and lower body are active, decoupled and independent, with the purpose of pivoting and steering, i.e. turning the skis. Also, counterrotation is the simultaneous rotation, in opposite directions, of both the lower and upper body, for the same purpose.
In the essentials framework1, the view is that the lower body simply follows the skis, which turn because of other movements (like tipping) and the only rotational movement of interest is counteraction, as in counteracting the turning of the ski, by rotating the upper body the other way.
As opposed to using rotational movements to make the skis turn (pivoting, steering), the essential movement of counteracting is more similar to coiling, to maintain rotational balance, separate the turning ski from the "stable" upper body and create alignment, balance and performance.
Note: The dictionary definition of Counteracting is "to act against in order to reduce its force or neutralize".
As any countering with the upper body requires tension in the lower body, the question is really one of purposeful lower body rotation and the intent of it. Coiling itself is sometimes seen as an upper body action or a lower body action, the difference between steering and coiling being often blurred, see Coiling and Steering and coiling for more details.
The essentials of flexing and extending are independent movements of each leg (ankle, knee and hips), so there are a few combinations, while in the planes of movement approach, the flexion and extension usually refer to both legs simultaneously, as in "moving the center of mass towards/away the base of support".
The essentials' point of view seems better suited to describing high performance turns, where one leg is extended while the other is flexed (i.e. Long leg and short leg in race coaching parlance) as well as both Flexed release and engagement. Also, pushing as an active extension to release or to "move the COM" is discouraged in the essentials and often embraced in the "traditional" approach.
Other differences are that flexing is the only prescribed way to transfer balance and release (together with tipping) in the essentials, since active extensions are discouraged. Flexing the inside leg is used to increase the edge angle (engage) while flexing the outside leg is used to release.
Skiing skills are such things as "edging" or "pressure", a proficiency in a certain area of skiing. There are a few sets of skills that different organizations focus on. Skills usually form a framework for assessing skiing and guiding skier development (as "areas of competence"), not necessarily a framework for direct instruction.
Skills are in fact another perspective alongside the planes of movement, but fuzzier, since no specific movements are identified:
A benefit of using a movement framework is keeping both instruction and feedback in terms of what the skier can do as opposed to what the observer sees - which is often a challenge in instruction. Even so, there are certain skills that we track, evaluate and address specifically, such as Balance, Carving or Speed Control.
Break every move down into chunks, slow down practice and then put them back together The Little Book of Talent, Daniel Coye
Following modern learning theory, sports are taught in terms of their fundamental components: master coaches decompose the sport into fundamental components, which are taught and then recomposed back. These fundamental components are the movements of the respective sport, such as "lay ups, ball shooting, ball passing, ball handling etc" in basketball.
The essentials framework describes these fundamental components of performance skiing well and unambiguously. The limited subset of movements, arranged in a hierarchy, simplifies communication and teaching. Also, there are biomechanical advantages, as these movements (and sequencing of) are designed to not conflict with each-other and optimize "kinetic chain activation" and be less disruptive to balance (edging for instance is specifically achieved by tipping the feet).