I appreciate your continuous input. I was discussing this movement yesterday with a student. Snake Creeps Down (I wrote this sometime ago) is practiced (as you already known) within many Taijiquan Forms - performed at various heights (or "depths") - high, medium, or low. I remember a Wado Ryu Kata using this stance - but with the weight-bearing foot having the heal "lifted" during execution (see picture attached). I see similar stances (at various heights) in Shotokan. For me, Goju stances are similar to Shaolin - box-like, practical and strong (similar to Xingyiquan) - like our gongfu (Longfist) stances. As always, I am ready for correction - and thank you for it!
My teacher (Master Chan Tin Sang) taught Snake Creeps Down in three broad ways - "high", "medium", and "low". Low is always taught first - as it is the hardest to perform. It is also the most impractical to use in combat - so why do it? Well, the Heart Meridian as defined within Traditional Chinese Medicine (TCM) - is stimulated when the back of the leg is "elongated" and "stretched". This movement increases blood-flow to the tips of the toes from the inner organs, etc, creating a two-way exchange of "qi" (oxygen and bio-electrical) energy. Old qi is expelled - whilst new qi takes its place. I believe this concept has its origins in the "Neijing" [內經] "Internal Classic".
For combat efficiency (and flow), however, a practitioner must adjust this movement. The stance has to be high or medium (a type of "back-stance") before the elaborate hand application(s) can be effectively deployed. The excellent video-clip you forwarded, for instance, appears to remove the drop-down leg-position completely - and replaces it with a "Horse-Stance" mediated by a number of connecting (natural) stances (varying in height or depth). In our family system a similar approach is applied. When emphasising combat - the drop-stances are performed at a higher-level - as this allows for a more efficient transition between connecting techniques.
Hakka styles tend to be clever but robust - deliberately clashing with strength - when strength is dominant and over-powering. Even placing the weight onto a bent reverse-leg is viewed as "giving ground" - which is okay providing such an action is required. Qinna (擒拿) - "trapping and re-directing" is the principle directing the hand and arm movements. This interpretation will vary from style to style - and even between teachers within the same style.
For every official definition - a student is encouraged to seek-out a hundred more! The lead arm is the leather whip (which "snaps" from the shoulder-joint - although in reality only in the internal sense) - whilst the reverse arm (with its chicken-head-type block) - is the reverse arm. Re-directing, locking enemy joints, and up-rooting the opponent's legs and feet is precisely the applied function of the "Snake Creeps Down" methodology! The medical application is the "internal" - whilst the combat application(s) - is the "external". As I am currently staying in Galmpton (South Devon) - I am exclusively practising my Forms (Katas) whilst on holiday for the Festive Season.
All Best Wishes
Adrian