Yikes! Blocks? Multiple attackers? I agree that we in Taekwondo have this same technique, but I don't subscribe to this as being a block or a multi-person defense.
As to a block, it would usually follow to mean against a low attack - which would typically be a kick or a weapon - two very things that should never be blocked, unless one desires to end up in an arm cast in short time. And in any contrived scenario involving a kick or a weapon - or any other attack - one questions what the high back hand is doing? Or what the near leg is doing with its exposed and unprotected knee?
If the strike originated from high and ended up low, then the defense becomes a parry - not a block - or a grappling technique. And this makes more sense, because then we can interpret what the back high hand is doing. And, we can have a meaning for the stance, which the picture does not adequately portray except as approximately a horse stance, but which is different than in Taeguek Pal Jang (Taeguek 8).
And as to multiple attackers, any contrived scenario is always based on a lucky scenario not likely to happen. Were it to really happen, the focus for that unlikely scenario pulls attention away from more likely scenarios. It would force us to think as if we are stuntmen in a Jackie Chan or Sammo Hung movie: very improbable movements to unlikely encounters, however entertaining the result may appear. While we always assume we are attacked by more than one person, we must dispatch each attacker one by one.
The fact that we are in a horse stance according to the illustration, we presume our sole attacker is to our right or left.
As ColinSeligSmith mentions elsewhere in this question, this pose (and poomsae) can easily represent a catch of a kick:
You squarely face your opponent who throws a kick with either leg.
You step backward with your left foot, into horse stance. You are now off the line of attack, so you've met one prerequisite for self-defense.
As you step back, your left arm draws back and follows the kicker's knee (making this more a parry, and not a block); at the end of the kick's reach, you raise what is now the rear hand (the left hand), and because it's a fist, we assume we're grabbing something: the calf, ankle, shoe, or pants leg; and then we raise it high.
In raising the leg high, the opponent should be off-balance, and should be moving more or less uncontrollably toward you, passing to your front.
He is losing height as you are stretching him into sort of a split. This also neutralizes any attempt by him trying to follow up by using his hands to further attack. He might reach you, but not in any meaningful way; you are now meeting the next rule of self-defense: neutralize the attack.
Your ethics takes over from here, but, assuming the opponent is still dangerous, your "low block" is a strike to his head (which could be lower if he's lost his balance) or it could be a hammer fist to his knee depending on where you caught his foot, or it could be a hammer fist to his groin - especially if he kicked with is left foot and you both are facing the same direction; and in this case, if the hammer fist is not readily accessible to the groin, it is your elbow - and not the fist - which can strike his stomach.
Taeguek 8 does not lower the high hand as if in a strike, so, for Taeguek 8, this would not be the best interpretation. However, the illustration - without context of what happens next - could infer this as one possible scenario.
In this way, you've exercised the final rule in self-defense: you neutralize the attacker. So to recap: we've gotten off the line of attack; we neutralized the attack; we neutralized the attacker.
Another interpretation is similar, and does apply to Taeguek 8, and still applies to the illustration:
You square off; your opponent throws a kick, similar to before.
You back up (getting off the line of attack) into a horse stance as shown and as prescribed in Taeguek 8, which means backing up with your right leg.
As you back up, your "low block" is a parry: it leads (not blocks) the kick to where ever end it may land - low, medium, or high - but not on you, because you moved off the line of attack. No matter the assumption here, we don't have to assume anything about the attacker's kick quality; we just know that while we got off the line of attack, we did not give up distance to the attacker, and because the attacker closed the distance by kicking, we can assume that the attacker is in very close proximity to our right hand, which is executing a high "block". With his face right there, the "block" is a strike to the face/jaw.
Our own head is looking toward the low block, not the strike to the head. This might make sense: if there was another opponent to deal with, if he's behind your attacker (whose now been struck with your raised back hand), you need not yet worry about him, so no need to look in that general direction. The bigger worry is if you are sandwiched in between two attackers (one of whom you just took out his jaw), and the other is possibly coming at you. What you do next depends on the poomsae's interpretation of his attack, but, whatever you do, you keep your eye on him.
This is how we consider multiple opponents. We indicate our awareness of another opponent, and our positioning with regards to one: this is called strategy, and is one of the things our forms are supposed to teach us. That is precisely why we study - in great depth - of every nuance of the poomsae: eyeline, stance, open or closed hands, bending knees, foot placement - everything.
But in our forms - no matter the style - we can't presume the attacker will do anything warranting a specific attack: In Taeguek 8, we have the slow upper cut coming next. Why assume that is for the next opponent, when we might not yet have completely dispatched the current opponent? Indeed, the rising high block might just be the preparatory movement for a lock or a pin. In my studies, the slow movements are exactly that: preparations for a pin or a lock, which almost never require an explosive movement. Yes, a slow movement is not a slo-mo of an attack, like an upper cut; it's a movement meant to draw and control the opponent, and in which speed of the technique is meant to stabilize by keeping up with the opponent. Fast is not important, or even strength and power. Keeping up is the key, and that is representative of the slo-mo movements.
Explaining this further would draw away from scope the original question, so perhaps asking another question about what slow moving techniques are for is warranted. I only mention it because Taeguek 8 was brought up, and uses this as the very next movement to the pose being asked about, and its consideration has effect in Taeguek 8 application, and is not applicable to the question about the pose; and because of the mentions about multiple opponents.