This rules text covers a lot of what you're asking (Harm and Healing, P.21):
Damage is dealt based on the fiction. Moves that deal damage, like hack and slash, are just a special case of this: the move establishes that damage is being dealt in the fiction. Damage can be assigned even when no move is made, if it follows from the fiction.
HP loss is often only part of the effect. If the harm is generalized, like falling into a pit, losing the HP is probably all there is to it. When the harm is specific, like an orc pulling your arm from its socket, HP should be part of the effect but not the entirety of it. The bigger issue is dealing with the newly busted arm: how do you swing a sword or cast a spell? Likewise having your head chopped off is not HP damage, it’s just you being dead.
The separation in your scenario should be clear, in the fiction your opponent has "Hit you with his warhammer" and as a result "you take 5 damage" is damage dealt based on that fiction.
If that warhammer was instead viciously aimed at the character's kneecap, they're not going to be walking for a while, but the injury is probably less directly life-threatening that a blow to the chest, so consider reducing the HP damage. However, that character now has a broken knee, if they try to make a fictional action which this could impede, remind them of that and ask how they cope with their injury, for example:
GM: Okay, this just got ugly, the Orc smashes your knee in with his warhammer. You hear a loud crack as the bone shatters and your leg buckles under you, take 5 damage. What do you do now?
Player: I'm gonna make him pay for that, I make a mighty swing with my greatsword, I'm going to take his damn leg off.
GM: Your leg's completely shot and that sword's heavy, how are you planning on keeping your balance?
Player: I'll just keep my weight on the other leg.
GM: Alright so you're attacking while balancing on one leg, roll to Defy Danger with DEX to not fall flat at his feet.
Player: Seriously? Can't I just roll Hack and Slash?
GM: Nope, you're not engaging him in a melee if you fall flat on your face before you can land a blow.
Player: Alright... *rolls dice* Damn! That's 5...
GM: Ouch, too bad. Your overzealous attack causes your leg to give way completely and you fall flat on your face at the Orc's feet. Grinning maliciously he raises his warhammer above his head to deal the final blow, what do you do?