But, in analogy, they don't get to select their DB after the attack roll, but in the space of time before the effectiveness of the attack has been determined. To select DB after the attack roll has been announced would be the same as deciding how hard to block after the punch has landed.
That's a misinterpretation of the mechanic both as described, justified and implied.
I'm not looking at the description/justification/implication of the mechanic, I'm looking at it's ultimate effect. What you say the mechanic is about, how you justified it, aren't as important as what the mechanic is actually doing.
In 'real' life, a competent combatant will see/know/feel whether an attack is weak or strong and apply effort according to need.
No. a competent combatant will be able to _GUESTIMATE_ the effectiveness of an attack. They not know how effective it is until it lands. They will not feel how effective it is until it lands. And, as for seeing, what you see can be deceptive -- there's a whole part of fight training and experience that goes into how to throw feint punches and such in a way so as to force an over-reaction from the defender. That's part of what I'm talking about -- in real life, the defender doesn't ACTUALLY know how effective the attack is until it's too late to change their defenses.
Before that, during the time where they can still change their reaction, they don't KNOW how effective the attack is, they're estimating it, guessing it, based upon training, observation, and experience.
In analogy, they don't know the final result of the attack roll at the point in time where they have to decide how hard to defend.
If the attack is weak - the defender can apply minimal effort to the defense and instead devote maximum effort to counterattack. That's the way it works in fencing, Judo, knife fighting, Iaido, Aikido, Jo, karate, gunfighting, wrestling, boxing, etc. I'm sure an SCA guy will tell you the same thing: Are you telling me that a combatant thinks this way: let's put 35% effort into attack and 65% into defense? Nan-unh.
I never said any such thing about a real life defender.
What I said was that they don't KNOW how effective the attack is before it lands, so they have to decide how hard to defend before they know the full effectiveness of the attack. You, in contrast, are implying that these combatants have true precognition -- not "experience that gives them a better estimation of attack effectiveness", but "actual sense of the future which tells them that this is a bone breaking bunch". And to that I say "if that was the case, then there'd be some martial artists out there who had gotten a million dollars from the Amazing Randy, for having proven ESP to be a scientific reality".
The difference between what you're saying and what I'm saying is this:
By giving the defender the total result of the attack roll, before they have to commit a defense, you are telling with absolute certainty how effective the punch will be before the punch lands. You say this type of ESP combat precognition is realistic and part of every martial art. I say "no, it's not".
Having studied four of the fighting styles you listed, I am quite confident in saying that none of them, not even the most spiritual and touchy-feely of them, gave me ESP. They gave me different ways to try to anticipate the effectiveness of an attack, but they didn't give me true, concrete, absolutely certain, foreknowledge of the outcome of the attack before I had to react to the attack.
I say that this is the case because the defender must commit to their level of defense (in real life, in the game) before the punch lands (otherwise it would be too late).
I am saying that the defender doesn't know how effective the attack _ACTUALLY_ is, with certainty, until the attack lands. But, you're asserting that a combatant knows this information with certainty before the attack lands. I'm saying that's absolutely not true. They can guess it and estimate it based upon observing their opponent, based upon experience in fights, and based upon their training. But it's a guesstimate. It is not absolutely certain information.
The attacker might pull their punch at the last possible second. They might not be putting full strength/effort into the attack. They might be throwing a feint so that they can follow up with a sucker punch. These are all things that the defender can guess, but doesn't actually know. The defender can be _wrong_ in their guess or in their estimate of the attackers intentions.
By giving them the actual outcome of the attack result BEFORE they decide how they will defend, you are telling them whether or not it is GOING TO BE a faint, whether or not it was a full strength attack, whether or not the attacker is GOING TO pull his punch.... before the defender actually decides how hard to block. You are giving them this information with certainty (because they know the final result), not as their estimate, not as their guess ... with _CERTAINTY_.
Further, you've eliminated any capacity for the defender to be _WRONG_ about their defenses. What about when a head bob ends up putting you right into the path of a punch that wasn't quite going where you thought it would? It happens. People get knocked out that way. Defenders, even highly experienced and trained ones, are not always right. Yet, by giving them the final result of the attack, before they decide upon their defenses, there is no room for their defensive judgment to be wrong. What are they going to be wrong about? You've already told them the result of the attack roll.
What you are creating is a situation where every defender has combat precognition -- the full and absolute knowledge of the outcome of the attack should they decide not to act, and what amount of action they are required to take to cause specific different outcomes. Not just "highly experienced reflexes", but actual ESP like foreknowledge of the outcome of the attack. That's fine in a mystical hong-kong type martial arts story where a fighter might have been trained to predict the future... or similar settings with that type of element. But even then, it's only appropriate for fighters with that training/ability. But if you apply to SPAR across the board, you're giving ALL combatants that ability -- clueless newbies, incompetent pencil neck geeks, green recruits, mundane bystanders, arthritic grandmothers ... all of them. You've suddenly given all of them ESP level combat precognition.
I assert that that's what you're doing because:
1) at the time where they choose their level of defense, they already know the outcome of the attack. There is no more opportunity for the attacker to faint, pull their punch, etc. With the exception of the defender's reaction, it is a "done deal". Yet, as explained above, the punch has not actually landed yet - therefore they have fore-knowledge.
2) they can choose the damage level of the attack by picking an amount of DB that causes outcome A instead of outcome B. In effect, they have enough combat precognition that they are able to not just avoid an attack, but choose the specific outcome of an attack. Their absolute foreknowledge is allowing them to absolutely determine the future result of this event that hasn't happened yet.
And let me re-iterate: it is foreknowledge because a defender MUST choose their level of defense before an attack lands (why parry a stab that has already finished gutting you?), the point at which the DB is chosen is before the attack has landed. Yet, it is impossible for them to know, with absolute certainty, the full effect of the attack before it has landed. So, by giving them the latter at the point where they are deciding upon their DB, you are giving them foreknowledge. Not a guess, not an estimate, but ESP like foreknowledge.