So here's my take on the gory details....
I interpret the sequence
(i) ready position, preparatory to a 90º turn into a left front stance/down block;
(ii) 90º turn into a left front stance/down block with chambering retraction of the right fist;
(iii) movement into right front stance/middle lunge punch with chambering retraction of the left fist...
as a `minimal combat sequence', a set of practical combat instructions which assume a certain common kind of violence initiator, in this case a grab by the attacker to the defender's arm, shoulder or shirt (research by Patrick McCarthy, reported and documented in Bill Burgar's
Five Years. One Kata and J. W. Tichen's
Heian Flow System, suggests that a significant fraction of street attacks start with just such a move). The situation in question assumes that, as per the usual violent confrontation, attacker and defender starting off face-to-face. Assume for concreteness that the attack is initated by a righthand wrist grab by the attacker, to the defender's right wrist, arm or shirtfront.
The actual combat
moves encrypted by (i)–(iii) are decodable, on one bunkai decoding, as (i)'–(iii)' respectively, where (i)' is the combat interpretation of the
moves that are concealed within the
movements described in (i), etc.
(i)' the defender covers the attacker's wrist with his own right fist, or reverses the wrist grab—this is one of the very earliest SD techs we teach them—and in either case, simultaneously (a) twists the captured wrist counterclockwise, and (b) turns quickly counterclockwise pulling on the wrist—this is the concealed meaning of the apparent presentation of the defenders left side to the the attacker at the outside of the form (something that would be suicidal to do in a street confrontation, obviously)...
(ii)' followed by simultaneously (c) driving the left forearm against the attacker's now forcibly extended right arm just above the elbow (the lower part of the `chambering' phase of the `down block'), (d) hikite of the trapped fist by the defender's `chamberinging retraction' of the right fist (pulling the attackers right fist into a maximally extended positon to give the defender's arm pin on the attacker maximum leverage and trapping the attacker in position) and projection of the defender's full body weight forward into the pin via the front-stance movement, forcibly driving the attacker's upper body down and exposing their lowered head to the defender's upcoming counterattack.
Having driven the attacker into a lowered position via the arm pin described, the defender (e) quickly moves the left arm from its pinning position to near the defender's right ear into an arcing upward elbow strike to the side of the attacker's lowered head, continuing up to a position above the defender's right ear, and then lowers it in hammerfist strike or knifehand to major targets on the attacker's head: the carotid sinus or larynx. This downcoming strike can be subdivided at the defender's discretion into (e1) a spearhand elbow strike to the attacker's face (eyes are a good target) and (e2) the payoff hand strike to the selected target. The main lesson of the whole subsequence is contained here: if you can trap the attacker's arm while going outside, you own him and the fight is effectively over, assuming a correct continuation.
(iii)' A smooth muchimi shift of the striking left hand to a grab on the attacker's ear/hair/collar is immediately followed by a simultaneous (f)hikite retraction of the left fist to pull the injured attacker in and around and (g) a right-hand strike (maybe a fist, but I think a palm-heel strike is sounder) to the attacker's face with the full weight of the defender's body moving into a right front stance.
(iii)" An alternative analysis has (f)' the retraction translating to a grip on the attacker's ear or hair, pulling it back while at the same time (g)' the 'punch' supplies torque on the other side of the attacker's head so that a neck break results. Picture a good firm grip on the attacker's head with both hands and an extremely sharp twist (left hand pull while right hand pushes around) and you have the picture...
By this point, the confrontation should be effectively over (the blows must be delivered rapidly and with significant force, accurately placed, of course). So this three-move sequence has an internal logic to it that makes it useful to teach as a minimal combat episode. When I teach Kicho Il Jang, I therefore start with this sequence.
However, the actual logic of the sequence doesn't end there; in principle, the 180º pivot that corresponds to move (iv) and appears to merely set up a repetition of the same sequence on the other side is, on its most effective combat interpretation, something more than that: it's part of a throw which follows a muchimi transfer of the right hand's attacking role in (iii)' to a hold which plays together with the sharp pivot in the 180º rotation to unbalance the now very damaged attacker and set up the finale. On that scenario, the effective combat sequence doesn't end until move (v) on the righthand side of embusen line of the form. The problem is, pivoting is a tricky enough skill for beginners that it's probably better to teach that separately and drill students on it so that they get the feel of a balanced, rapid reversal of orientation using the ball of their pivoting foot, which usually takes a few classes to `get'. In my own instruction, I just take students from (i) to (iii) as I indicated above, and add in the followup throwing and subsequent techs later.
I'm still working on a plausible analysis for the three punches up the line from moves 6 through 8. Taken literally, the sequence strikes me as absurd; but I suspect that at least one of those 'punches' corresponds to a neck twist of the kind I suggested above.