How do you decide?

The PCs have just suffered an attack, ignored a threat, or rolled a 6- and you think you're going to do harm. Do you deal damage? Do you have them mark a debility? Do you inflict a problematic wound and if so, how bad do you make it?

There's no hard-and-fast rule, but you should follow your principles and make a GM move that flows from the fiction. Often, you'll pick a GM move (like put them in a spot or a monster move like bash prey to the ground, senseless), and the specific fiction of your move means that they also take damage or mark a debility.

Other times, you'll know that you want to do something harmful, but won't have a specific move in mind. In that case, have the player roll the damage taken and let that inform the nature and severity of your move. See Running the Game for more guidance.

Keep in mind the tags of the attacks involved. Messy attacks are especially destructive, and should often hurt them or use up their resources (destroying shields, weapons, or gear). Forceful attacks knock victims around; even if they do no damage, they should still change the momentum or positioning of the fight.

When the PCs or followers deal damage to monsters, use the description of their attack, the amount of damage dealt, how many HP the foe has left, and the tags on the attack to inform the fictional results.

Announce trouble (future or offscreen): "Andras is white as a sheet, and that snake bite is red and swollen and you see angry red lines following his veins up his arm. What do you do?"

Reveal an unwelcome truth: "Oof. Yeah, Blodwen, you peel the bandages back from Garet's wound, and… it's not good. It smells bad, the flesh is kind of black, and there's a greenish pus. It's infected, and it's going to get worse. What do you do?"

Put someone in a spot: "It claws at your face before you hurl it away. Take d6 damage. You're still up? Okay, I think it gashed your forehead and blood starts pouring into your eyes. You can't really see, but you hear it scuffling about, getting ready to pounce again. What do you do?"