I broke down Player Agency into some conceptual units - Fact, Yes-And, Yes-but, Gambit/Gamble. It is in my Definitions, adding more of the 3rd Gen conventions and concepts in the ORPG.
Player Agency allows players to make Narrative Facts about their Characters within the consensus of the rules and its assumptions, while the GM makes narrative facts about the world (same exception applies: consensus with rules sand its assumptions). Facts about the Character, the GM can only Yes-and or apply rules and their consequences (like the character takes damage or is afflicted with a psychological condition).
The GM wants to afflict a character, he has to go through the Player and the convention of the rules - He has an NPC attack the PC or a Boulder falls on the character. Narrative Facts don't just Happen to PCs, the Player has conventions in the form of Defense, a chance to protect himself.
An important concept is the procedural - that Players have agency over their character but it doesnt mean their character is immortal or cannot "Play along". The Rules act as implied convention. Without going deep into Philosophy - we are all telling a story and playing a game, every action can be reacted.
Dangerous Facts. With every Action, there is a Re-action or unintended consequences. What if the Player can turn a fact on the GM or another player. When a player reacts - making his defense, his perception roll, etc... what if he can make a Gamble, take the opportunity to sacrifice odds of success in return of being able to make more Facts.
NPC misses, creating an opening. Dodges the Boulder while pushing another Character in its path.
Fact, Narrative (Fact)Simply this is something declared as fact and true in an RPG. As a Fact, it has to be consistent through out the game, and accepted by everyone. A fact cannot be changed without some narrative justification – which means GM and Players play out the fact. But the Fact is also subject to details and more specificity.
A fact affecting any "Character" gives that Character a chance to resist. An Attack is an example Action, and Narrative Fact, that is trying to affect another Character. The character resists, with his defense.
This distinction becomes important when the Player or GM makes Gambles/Gambits, sacrificing their odds for success to have more control over the outcome. GMs also bribes Players with rewards to alter their Facts.
Many game system mechanics automate and simplify the informing of all those playing of various results, consequences and facts. These can be “altered” or “specified” by more Facts that come into play. RPG is a game of consequences, and many rules work to provide consequences to actions - the more consequences the more options for actions.
Exception: Characters can react and counter Narrative Facts - PC or NPC.
Limiting Facts: The only limit is the ability of a Character (a humanized concept) can attempt to counter or resist a fact imposed on it.
Yes-AndYes-And means the GM or Player agrees with the stated Narrative Fact and adds another dimension to the fact. Basically countering with another Fact, which may complicate or compliment. It depends on the narrative. Adventures are always filled with complicating “facts” and consequences to actions.
Yes-ButYes-But means the GM can agree with the Narrative Fact on a condition. Unlike Yes-And, Yes but gives either the GM or Player the ability to turn down a Narrative Fact.
At default the GM has this power but players can have this when they make and win a Gamble/Gambit.