How does concentration work?

[Yqatuba]

Maybe this is just me being dumb but I can't find an answer in the core book. The glossary says it means the thing "requires a degree of concentration of mental discipline" but doesn't say what mechanical effect it has.

[HammerJack]

The concentration tag doesn't have a DIRECT mechanical effect. Other rules, like Rage or Disruptive Stance, use the concentration tag, indirectly.

[Lost In Limbo]

It's just a tag that other things can interact with.
For instance, Barbarians using Rage can't use any actions with the concentrate trait, and a Fighter with the Disruptive Stance feat can use attacks of opportunity against someone using a concentrate action.

Many descriptor tags don't do anything by themselves in Pf2 until there is something for them to interact with. Emotion is another good example, the emotion tag on a spell or effect doesn't mean anything until you try to use it against someone with resistance to emotion effects.

[Yqatuba]

What if a monster has shapechange with the concentrate tag? Does that mean it can't concentrate on something else if it's currently using the power?

[Lost In Limbo]

No, it just means that the action itself to shapechange has the concentrate tag.

Looking at the barghest for example, it is one action for them to Change Shape which includes a bunch of tags, notably concentrate. That means if a barghest tries to change shape while within reach of a fighter in Disruptive Stance the fighter can use an attack of opportunity and potentially disrupt that action.

I think I've heard that 5e has a concentrate mechanic similar to what you're talking about, but Pf2 doesn't. The Sustain a Spell action is a little similar but less restrictive.

[tivadar27]

Yeah, so there's a few differences here. First, the basics is that "concentration" in and of itself isn't really a thing anymore. Actions have the concentrate tag, and enemies reactions may be triggered when you use those actions (typically within reach of them).

This is different from 5e, where things with the concentrate tag must be maintained and you can only maintain one thing. This is closest to the "sustain" ability in 2e. Spells that are sustained require you to use an action to sustain them on your turn or they end. However, you can sustain multiple spells, it just requires more actions on your turn.

Finally, this is even different from the playtest, where if you got hit while (sustaining I believe?) a spell, your spell would be disrupted if you took a certain amount of damage. That now only happens, to my knowledge, when one of those reactions we talked about above critically hits.

[HammerJack]

More accurately, losing the spell can happen under any condition where the reaction states that it can disrupt the triggering action. Attack of opportunity disrupts on a critical hit (if the trigger is a manipulate action) by default. Attack of opportunity with Disruptive Stance will disrupt a concentrate or manipulate action on a regular hit. Other reactions may vary in whether they disrupt at all, and what condition they need to meet for that to happen.