Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

So! My char wound up instructing a unrefined fighter in the philosophy of fencing. This means i gotta figure out ways to talk certain class mechanics icly.

Explaining what Duelist Int to AC is was easy. Dex to ac is basically reaction speed to either evade or parry or blocm. Int to AC is predicative power and superior positioning - in essence, it allows the duelist to use their advanced understanding of the science/philosophy behind fighting and how people act and react to react to a movebefore the opponent even realized they wanted to perform that move. Think how a very experienced chess player can easily predict how a lesser player might play.

Dex to AC makes sense why AC is impeded by armour - It getd in the way, slows one down.

It however eludes me why, with my interpretation, int to ac faild.Especially since bladesingers can do it in armour (although with half as much luck).

This is neither a suggestion or complaint - oocly i get why it is like the way it is. I just want to figure out how to ICly explain this to my char's student.

I think just because you have the brains to come up with superior positions and prescience about enemy movement does not mean you can put yourself into those positions while encumbered with weight downing you.

This is something I have thought about in real life, but I am only an amateur practitioner of HEMA and my cultural martial arts so my experience is an anecdote at best, so take my opinion with a grain of salt. And I'd like to emphasize here what I am talking about is not about being dextrous its about prediction. Most of my sparring in padded gambeson, there is a slight lag on acting speed, because when I formulate my plan of action in my head during the heat of the spar, I subconsciously plan for a state in my best condition aka with perfect maneuverability which my cumbersome gambeson inhibits me from reaching. That creates a very split second delay on how to act upon my plan. I am not sure how to completely word it, but I guess I would say, when I am encumbered with the weight I have put my plan through a filter to adjust my ideal condition to my current condition.

Again English escapes me sometimes but perhaps if you flair this line of thinking, it might help to explain your disciple about the situation.

The easy answer would be to simply say that (much in the same way that having a certain amount of agility is impeded by armor) if you burden yourself down, you cannot position yourself in an optimal manner, and thusly, regardless your predictive capability and swordplay, you're limited to how quickly you can take the ideal position to avoid a blow.

Overall, D&D is a very abstracted system. As such, it doesn't work too well when put through a fine filter. Considering actual historical martial arts, agility and reaction speed is still needed whether fighting in full armor or without. "Max DEX to AC" is more of a balance mechanic, and and such, I typically avoid trying to explain non-magical game mechanics with lore when it can be avoided. For some reason, when trying to do so it seems to feel like bulls**t hand-wave explanations.

Google Image Search for "rapier duel" suggests that armor was not used for such in the real world even though it was readily available. I personally always took this PrC as being modeled off of the real world gentry of yore's preferred fighting style. So the answer to OP's question may be a matter of historical record.