When a character takes damage, they receive a new Injury. Minor injuries, while still serious, heal relatively quickly and do not impair the target much. Severe injuries leave Battle Scars, disable the bearer, and are much more difficult to treat.
Severity of Injury = Outcome on attack roll - Outcome on defense roll (if applicable) + Weapon Damage - Armor. If the attack Outcome - the defensive Outcome is less than or equal to 0, that means the attack failed, and thus no damage is dealt (calculate this before Weapon Damage and Armor are taken into account).
Characters who suffer an Injury with Severity greater than their rating in Wits must roll Body, Difficulty 9. Wound penalties from the new Injury apply to the roll. If they fail, they are stunned and lose their next Action.
A stunned character loses their next Action, which will be the current round's Action if they haven't already acted, or the following round's if they have.
Stuns are common and incredibly dangerous. If you find yourself dying to stuns, consider finding a way to get a numbers advantage.