Adventures In Eberron
Action Points are supposed to represent a hero’s ability to overcome critical situations, where failure or success can decide the fate of the world.
Players start each game session with one Action Point, and they don’t carry over to the next session. You can earn extra Action Points during a session by performing heroic acts, something valorous and altruistic. Beating up goblins isn’t very heroic. But taking hits for a friend, rescuing villagers from a burning tavern, surrendering to a villain in order to save hostages, allowing a villain to escape so you can stop a monster from killing innocent bystanders, those are all examples of acts that should grant extra Action Points.
Saying something inspired that makes everybody else at the table laugh or cheer you should also earn you extra action points.
The players also earn Action Points when the DM changes the rules in favor of the enemies. Villain escapes automatically? Party gains free Action Points! Player automatically fails save against a trap so the plot can advance? Party gains free action Points! Enemy gets a free surprise round? Party gains free Action Points! NPC uses an Action point? Party gains free Action Points! Also yes, only PCs get Action Points by default.
But what to do with those Action Points? Well, you can spend them as a free action that doesn’t consume any time, unless a rule says otherwise. When you spend them, you get to roll a die and add the result to either any 1d20 roll (after knowing how much you rolled, but before knowing if it was enough to succeed or no), or add to your AC until the start of your next turn. You can also roll a second save to shake off an ongoing effect on yourself such as Dominate Person, or gain an extra Standard or Move action . Finally, you can spend an Action Point to recover HP. But how much you roll you may ask? Well, it depends in your level.
PCs of level 1-4 roll an extra 1d4, or recover 1d8+1 HP.
PCs of level 5-8 roll an extra 1d6, or recover 2d8+3.
PCs of level 9-12 roll an extra 1d8, or recover 3d8+5.
PCs of level 13-16 roll an extra 1d10, or recover 4d8+7.
PCs of level 17-20 roll an extra 1d12, or recover 5d8+9.
You can spend multiple Action Points per round, but no more than one for each kind of benefit per round.