Advantage--and--Disadvantage
Advantage usually means: roll 3d6 and keep the two highest dice. Disadvantage usually means: roll 3d6 and keeping the two lowest dice. If there’s a tie for lowest or highest, discard one of them; it doesn’t matter which.
For damage rolls, advantage/disadvantage apply only to the “main” die. If a player gets any bonus damage dice, those dice get added after you pick the highest/lowest die. For example, the Heavy (who has advantage when they deal damage) rolls a 10+ to Clash, and chooses to strike hard and fast for +1d6 damage. They roll 2d10, deal damage equal to the highest of those two dice, and deal +1d6 damage on top of that.
Advantage/disadvantage don’t “stack.” They’re binary—either you have them, or you don’t. For example, if someone Seeks Insight and gets a 7+, they get advantage on their next roll to act on the answer. If someone also Aids, they can’t get advantage again. They roll 3d6 and discard the lowest die.
If someone has both advantage and disadvantage on the same roll, the two cancel each other out and the roll is made normally. By default, it doesn’t matter if you have multiple sources of advantage vs. a single source of disadvantage (or vice versa). But if you and your players really want to get into counting how many instances of advantage or disadvantage a player has, you can. It won’t break the game, but it might slow it down.
Players might have advantage on a roll because…
… a move (like Aid or Seek Insight) says they do; … some sort of equipment or arcana gives them advantage; or … you offer an opportunity to make a particular roll with advantage (but see the sidebar below).
Players might have disadvantage on a roll because… … a move (like Interfere) says they do; … they have marked a debility (see page XX) and roll one of the affected stats; … some sort of equipment or arcana imposes disadvantage; or … you reveal a downside or tell them the consequences, and it involves rolling with disadvantage (but see the sidebar on the opposite page).
Adjusting difficulty
In many RPGs, the GM routinely applies bonuses or penalties to player rolls to reflect the difficulty or ease of their tasks. In Stonetop, you can apply advantage or disadvantage “on the fly” to do the same thing, but do so sparingly—it undercuts the impact of moves, arcana, and debilities that specifically grant advantage or disadvantage.
If a character’s action seems particularly easy or difficult, consider these instead of advantage or disadvantage:
Are they even triggering a move? Can you just say that they do it, or know it, or figure it out? “No, don’t Clash, just deal damage.” Or, can you just say “no” and tell them the requirements for making it work? “You can’t make that jump with all your gear, you’ll need to drop down to a light load first.”
Can you adjust the risks? If they have a strong position, make it clear that the risks are less. “Suffering his attack just means that he’ll get inside your guard.” If they’re in a desperate position, tell them just how bad it could get. “On a 7-9, you’ll have one last chance to catch the ledge, but on a 6- you’re rolling Death’s Door.”
Can you adjust the scope? If things are easy for the PC, let a single roll accomplish more, or reveal more, etc. If things are hard, break the action down into smaller moves, each of which can only accomplish part of their goal.