Your character's Expertise (abbreviated Exp) represents her training, drills, and education received throughout her life to pick locks, juggle, identify, forge, investigate, track, or whatever else method of applying learned skills to solve her problems.
Whenever a character attempts to do something, but the outcome is in doubt, an action check is used.
action check = 3d6 + 1d6 per 3 in corresponding attribute + Boost die
A 6 on a roll represents a success for an untrained character, and a 5 or a 6 represents a success for a character with the appropriate Expertise for the action.
Generally, one 6 is enough for a success, although for difficult actions, the GM may require two or more 6's for a success.
Note that not every action check is going to have a corresponding attribute or expertise.
Boost dice pool
Every character has a dice pool that represents a reserve pool of power he can use to focus on the task on hand. The boost dice pool is 3 + 6 per each Expertise the character is trained in. When an action check is called for, a character may boost the action check by expending dice from his dice pool.
The boost dice pool regenerates after a long rest. (Typically one day, but your GM may renew the dice pool as often as he wants)
Jordan attempts to balance across a plank across a gorge in combat. Note that if time wasn't an issue, Jordan could take his time across the plank and guarantee his success, and there would be no need for an action check.
Jordan has an Agi of 7, and he is not trained in Acrobatics Expertise. His character is trained in 3 expertises, bringing his total boost dice pool to 21. He decides to spend 5 dice from his pool to boost this check. Therefore, Jordan rolls a 3d6 + 2d6 (1 per 3 Agi) + 5d6 (boost) for total of 11d6 for the action check, and counts the number of 6's to see if his action succeeds.
A character may voluntarily reduce their action check by six dice to treat their result one success higher.
In the example above, Jordan may instead roll 5d6 for his action check and add 1 to his number of successes.
Complex action checks
Certain actions may require multiple successes over multiple rounds to succeed.
For example, the party is fleeing from hungry ghouls when they discover that the entryway is caved in with debris, and the GM may require a total of 10 Athletic action successes for the players to clear the path.
Cooperative action checks
Certain actions may be attempted by multiple characters working together. In this case, one person will make the action check, and for every additional person assisting, adds 1 dice to the action check roll. Any of the players assisting may boost the action check. If at least one person has the corresponding expertise, then all of the die succeeds on a 5 or 6.
For example, Peter the bard is dying after being attacked by savage monsters, and his team is trying to stabilize him. Their merchant, Agnes, has an expertise in Medicine, and their two other teammates are assisting her. Her teammates choose to expend 6 dice from their boost dice pool to help. Agnes rolls 3d6 + 2d6, for each of her teammates, and +12d6, for her teammates' boost die, and counts the number of 5 and 6's for success.
GM may limit the number of characters that can assist with an action check.
List of Expertises
An expertise represents a special training or experience the character has. In addition to helping action checks, each expertise has a special combat perk that may be used during an encounter.
You are able to slide, dodge, weave, backflip, and parkour your way through most obstacles.
Combat perk: You know how to weave and bypass around enemies in combat. You may move through enemy squares as if they were allied creatures.
You are a master of using the arcane bits of dust and grit to create useful alchemy products.
Combat perk: You excel at discovering alchemical ingredients in strangest of places. Whenever you roll for loot in combat, you may always discover alchemy essence instead.
You are a powerhouse, able to smash, shove, climb, and lift the obstacles in the way.
Combat perk: You may Push 1 allies and enemies by moving into the square they are occupying. You may not push creatures that are larger than you.
You have studied the otherworldly creatures that have arrived here through the Nightmare portals.
Combat perk: You know where the weak points of most demons are. You receive +3d6 weapon damage against demons.
You know how to work with, tame, calm down, teach, and manage animals.
Combat perk: You know where the weak points of most animals are. You receive +3d6 weapon damage against animals. (Excluding intelligent animals)
You can tell when a scammer in the market trying to pull you a fast one, or have a sixth sense telling you when to duck.
Combat perk: Once per combat, you know ahead of time the result of a future dice roll.
For example, you might be trying to decide whether to attack a battered enemy one last time, hoping to take it down, or to retreat behind a door and lock yourself in. You can use the Insight expertise to roll your weapon damage as if you attacked, and see if you would have defeated the enemy. If the roll is favorable, you may go ahead and attack for the rolled damage, but if the roll if unfavorable, you may decide to retreat behind a door instead!
You have good attention to details, able to locate hidden traps and doors, as well as estimating how much armor or health an enemy might have.
Combat perk: You know the amount of hit points and armor value a visible enemy has.
You know useful information from books, articles, journals, and reports.
Combat perk: You are able to identify special abilities or qualities of the enemy monsters in combat.
You know how to diagnose, identify injuries and manner of death, and apply first-aid for intelligent humanoids.
Combat perk: You know where the weak points of intelligent humanoids are. You receive +3d6 weapon damage against intelligent humanoids.
You keep up to date on the news in the locality, aware of who's in charge and who can provide services, and know the local customs and where the good hospices are.
Combat perk: You are able to identify special abilities or qualities of the enemy NPCs in combat.
You have trained your eyes and ears to locate camouflaged objects and creatures.
Combat perk: You go first even in an ambush.
You have studied the art of magic, and you have memorized various diagrams, gestures, and equations used in spell formulas. You are able to deduce a spell's effect, based on the incantations and spell circles used.
Combat perk: A creature is unable to pay action cost of a spell if it's in your melee range.
You have mastered the art of disappearing out of sight, using whatever tools and means you have available.
Combat perk: You may go into stealth even while being observed.
You know how to forage, hunt for food, identify weather patterns, track trails, and avoid natural hazards.
Combat perk: Once per combat, when you are about to take lethal damage, you instead survive with 1 hp. You lose 1 action in your next turn.