Mechanically, if the level-up happens while the party is out adventuring, nothing prevents any character from advancing any of the existing skills by a number of ranks.

That means that the low-intelligence fighter can theoretically gain ranks in Knowledge(Arcana) or Spellcraft mid-dungeon and become moderately competent in it. With no trainers, no materials to learn from, no time spent, no nothing.

Furthermore, if the skill being learned is Linguistics, than the character also gets to learn a whole new language. And the ancient obscure ones are no harder to learn than the modern ones.

I have encountered GM's that will outright ban that mechanical possibility because they consider that implausible.

What is the best most defensible in-story justification that can be given for characters learning to use trained-only skills on level-up?

The quality of the justification can be hard to measure, so I'll try to outline some arbitrary constraints:

  • We'll say that the justifications that require prior in-story preparation by the player are worse than the ones that can be used even if the player decides to take the rank the moment the level-up occurs.

  • The setting-independent justifications are better than the ones relying on specifics.

  • General ones that can be applied to all skills at once are better than arrays of different justifications for different skills.

  • The less "But what about/if ?" questions the justification raises - the better.

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    \$\begingroup\$ I don't think you are understanding what "trained only" means for skills. Check out the srd for skills. Trained only means that you can only use the skill if you have ranks, not that you need to get specific training to get/increase your ranks in it. You don't need any more justification for learning Spellcraft than you do for increasing Perception. \$\endgroup\$ – GreySage Feb 27 '18 at 18:20
  • \$\begingroup\$ Well, there's still a leap between "can't use it ever at all" and "may be the best in it if the dice are right" that you make without training. Kind of meant that aspect of it. \$\endgroup\$ – Nox Feb 27 '18 at 19:10

As you alluded to, gaining new skill points is not adequately addressed by rules in terms of "How do I justify this?". You level up and, maybe, wait for the end of the in-game day (or end of session) to "activate" that level up.

"How did the fighter suddenly go from ignorance to proficiency in spellcraft?!"

I prefer that: the character has always been working on this thing. Back in the first village, Fighter McFighterson grabbed a "Dummy's Guide to Spellcraft" and has been getting through it for the past X-levels. It was a bit confusing and he was too embarassed to ask the wizard about it, but it's finally started to "click".

If you're looking for a solution that "requires no prior commitment", then some of that might be hard to find. With a bit of lore-smithing:

  1. Many skills could easily be described as a latent knack the character never realized. "huh, who knew I was so good at picking locks?".
  2. Some sort of external influence (A diety, a Patron, or any similar creature) has bestowed/unlocked this talent.
  3. The character has refused to show/use the skill for some reason. Maybe they didn't want the fighter to know how far they could jump, just in case they got into a fight. (might fit into the first category)

P.S. - as a note: "class skills" in pathfinder require little-to-no explanation because they have the characters implied background to fall back on. McFighterson waits until level five to put points into acrobatics? His class implies (heavily) that he had been doing that to some extent before you (the player) came along and started controlling him.

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    \$\begingroup\$ The Dungeon Master’s Guide for 3.5e (which Pathfinder is based on) explicitly suggested this approach, and also covered a number of other issues as “background activities” that don’t explicitly come up in game but are assumed to happening (care and maintenance of equipment is another big one). \$\endgroup\$ – KRyan Feb 27 '18 at 16:43

There are several narrative solutions which I am going to list

  1. You have been training that skill for a while now and you recently finished developing the skill to the degree you can use it.

  2. The wizard of the group taught you the trained skills along the way.

  3. As you are constantly exposed to people with different knowledge due to your adventurer life, you learnt enough to be able to guess certain things.

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Don't assume that there is (or should be) a narrative justification in a system that not based on narrative justifications, such as Pathfinder. There isn't one. This is GM fiat.

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