Flexing and extending of joints are how we move and stay in balance. Traditionally seen as just a means to control pressure, that's not the entire story - here's some of the biomechanics around flexing and extending (long leg and short leg).
Generally we will talk about the flexion and extension of the legs, i.e. getting short through flexion vs getting long via extension. The joints of major interest to skiing are:
These all contribute to the leg extending and flexing (i.e. getting short or long), while staying in balance:
As a movement, many frameworks consider vertical movement as an entire category 2, described as changing the position of the COM vs the BOS (base of support) in the vertical plane, as a "holistic" movement of the lower and upper body.
However, we have to remember that in skiing, the planes of references are tied to the skis, see Vertical movement in performance skiing.
Often, flexing/extending is seen as a pressure control mechanism 3.
This is a somewhat more comprehensive view, and also the common view in racing coaching, because it also includes not just the up/down movement of the COM and pressure management, but also Long leg and short leg, which is an important aspect in racing and performance skiing.
So, while flexing both feet moves the COM down, flexing only the outside leg creates a release and flexing only the inside leg transfers weight to the outside ski and helps establish inclination and allows tipping into the turn. Also, in the Essentials framework, the extension of the outside leg is not "active" to impulse the body "up" but passive, to maintain contact with the snow, flexing being the preferred release mechanism instead.