I mentioned this morning I'd take a deeper look at Doug's post and do so comments on it here. So, here they are.
Stealing Doug's writeup, but with my own comments:
Red: Attack, a swing to the head. Maybe a Committed Attack? It would explain . . .
White: Parry 
White: Deceptive attack. Swing to the torso.
Red: Cannot parry; must dodge. This does not go well and he's hit.
Red: The hit hasn't registered with him yet, and he's already made up his mind. Attack, Committed, to the head.
White: Drop to knees and parry 
Following Doug's numbers, as well:
 I think that attack by Red Socks that leaves his sword high can be one or both of the following:
Committed Attack (Determined)
A critical miss. Either because of a stellar parry by White Socks (maybe, probably not) or just a poor roll. One of the results on the Critical Miss table, with a roll of 7 or 13, is "You lose your balance. You can do nothing else (not even a free actions) until your next turn and all your active defenses are at -2 until then."
That squared-up stance he takes after a failed attack? That doesn't seem to be part of the attacking strategy so much as a miscue. I've seen fighters doing really well, then suddenly put a foot wrong and leave themselves unable to stop a strike.
Note that 7 and 13 say "nothing else" - so a -2 to defenses, and loss of the expected and planned for Retreat makes it a 5-point swing in defenses. Not only that, but a 7 or 13 result stops a turn dead, which means anything you intended to do after the strike that failed doesn't come off - no step, no more attacks (if it was part of a multiple-attack turn), no dropping a weapon, nothing. You just have to stand where you chose and take any counter.
Now, any of that could have resulted from the parry if White rolled a critical success. It just goes to demonstrate that GURPS is positing dice rolls giving results, not necessarily saying the rolls individually tell the whole story. Aparry that doesn't look so good but which is good enough, and then which causes the attacker to be unable to respond effectively (in other words, Critically Miss and have bad stuff happen) can be a Critical Success. Probably a Critical Failure, though.
Or, it could be a Riposte Martial Arts, p. 124) - taking a slightly riskier defense to leave the attacker open. Although I'd say it doesn't really resemble what fencers call a riposte. It does fit into the GURPS terminology, however - Riposte as a term of art, describing game effects. That's iffy because White Socks doesn't seem to be taking a risky defense to open up Red Socks to retaliation, nor does it looked like a rushed defense (another way to be taking a GURPS Riposte.)
 This can also be a Counterattack (Martial Arts, p. 70). It's something you see a lot in sword manuals - the parry-counter, the masterstroke that delivers a blow while leaving you covered completely and your foe open, the immediate response. Counterattack is one of those techniques you need to train to have it matter (a quirk of the Deceptive Attack rules - if it was not so, it would either just be DA or be better than it), but it fits the style of German longsword really well based on the manuals. And clearly, how it's fought in modern reconstruction.
In any case, it's clearly a Deceptive Attack - probably through sheer speed, as the retaliation from the parry of the high-line strike is instant and fast. It's nothing "cute" but just hard to stop. That, per What Is . . . a Deceptive Attack? (Martial Arts, p. 111), is a wholly valid explanation for a DA.
 This is a good split-second to remember when you deal a potentially fatal blow in GURPS combat - unless your blow is so good, so well-placed, so effective that it makes the attacker unable to retaliate, you might still die. If you hit someone with enough damage that they will certainly die (from bleeding (p. B420), from lack of medical care, from eventually failing a death check) but they make a check to not be stunned and stay conscious in the meantime (or, luck forbid, act simultaneously with you) and kill you dead too.
I've had occasion (funny) moments when someone death a death-check causing blow, the "dead" person made the check, remained unstunned, remained conscious, and then acted with great consequence. While the players in my DF just go for torso shots not to "waste" damage, in my more gritty games you'd see a lot more arm and legs shots (yes, and eye shots) aimed at making retaliation impossible.
Don't forget Dying Actions (p. B423), too - if you're using them, the person might just get that blow in after failing a death check. If you killed your foe with a Committed Attack or All-Out Attack, well, you might just get to chat about it while you're walking down the long white tunnel together.
By the way, GURPS doesn't allow this exactly, but I do like the idea of dropping to one knee in lieu of a step back for a Retreat with a Parry. Take a knee, take a +1, and hope you don't need to move. It looks cool, too. It just sucks when you're kneeling and his buddy attacks, hammering your in your new, lower-defense posture.
Finally, I'm surprised Doug didn't highlight Setup Attacks, which also might explain the progressive opening of Red Sock's defenses. Perhaps the Setup Attack forced him into a limited line of attack, which made for an easier followup the next turn by the eventual winner?
It's All Fun and Games
None of this is a knock on the fighter we've been calling Red Socks. This is clearly some high level swordsmanship on both sides. But when you cut down to fractions of a second of movement, even a tiny error can cause a loss. I'd say most fight losses in high level combat sports result from one person's small error. We're simply taking an excellent slow-motion look at a combat sport exchange and showing what it could represent in visualization of game effects and what it could be modeled as using GURPS.
It's rare to get such a good example of concepts built deep into GURPS, so you'll have to forgive Doug and I for posting incessantly about it for a couple days. I promise I'm done with that one exchange.
Ironically, I don't do this kind of breakdown with MMA. This is because MMA is my sport - I have fought, I still train as if I might fight again, and I work on it constantly. So I don't really look at fight video and think of it in game terms, I think of it in application to what I do and how I can utilize or counter what I see. It's just not where I keep my head when I watch fights.
Incidentally, all of this reminds me of why you don't see this stuff in the movies - it happens too fast, the moves are too subtle and then too explosive. It's stunning in its execution but mostly unless it's slowed down you can't even see it happen. To an untrained eye the full speed stuff isn't going to look cool, anymore than watching high-level grappling is as entertaining if you don't know enough to spot all the stuff that's going on.