First off, all of edgerunner's answers are great. But I wanted to add some Dungeon World specifics:
Check p.19 and you'll see that 6- isn't "failure" - it's "trouble". The GM will say what happens and the player will mark XP. You are attaching non-DW simulationist ideas to DW mechanics by your supposition that 6- means "failure."
These principles can apply in all sorts of games, and have been used by GMs for years. If the PCs have to climb a fence, they're just going to keep trying until they succeed, right? So even in traditional games, many GMs will read "failed" rolls as a lack of some quality - not fast enough, not quietly enough, not without hurting themselves, etc., instead of just keeping them on the wrong side of the fence.
This is because failure is boring and stops moving the story forward. So you are correct, there is no plain-old failure in DW. It's not in the GM's agenda to make the PCs fail. There is no move for failure.
So the problem isn't that edgerunner's ideas are non-optimal, it's that your concept of what 6- means is wrong and that static failure doesn't exist in Dungeon World.
Expanding on 6-
From the text:
Generally when the players are just looking at you to find out what happens you make a soft move, otherwise you make a hard move.
Somewhere in Apocalypse World itself it says about hard moves:
make as hard and direct a move as you like
Early PbtA games like DW assumed you understood Apocalypse World. And this phrase is often tacitly implied in PbtA games even today.
6- means trouble as I said. The GM is free, on 6-, to make a move as hard as they like. That doesn't mean as hard as you can think of.
AW says:
It’s not the meaner the better, although mean is often good. Best is: make it irrevocable.
So while a 7-9 should substantially give the character what they wanted (they accomplish their intent even if their action created complication), on 6- you are free to deny the intent (the action still has to have consequences beyond "no" though) and in addition make a move as hard and direct and irrevocable as you like.
Climbing a mountain a soft move is "The boulders above you on the rock face begin to wobble as the grappling hook you've tossed up there sets itself. What do you do?"
A harder move is "The boulders have tumbled off the edge of the ledge and after hanging nearly motionless for a tiny instant above you, are now plummeting towards you, gaining speed every moment. What do you do?"
A really hard move is "The boulders are yanked free by your grappling hook and come smashing into you, tearing you from your narrow perch and scattering the contents of your pack into the yawning emptiness beneath. What do you do?"
In practice, you stick to the moves and use the list, until you internalize it and no longer need to refer to the list, but even them you continue to use it. Just like when playing a Eurogame with a board, you do what the game says, exactly, if you want it to function as advertised.
The GM's moves are multi-purpose. They:
- teach GMs new to RPGs how to GM
- teach GMs new to Dungeon World what their job is in this game
- help experienced GMs out of a rut, if they're in one
- when you're stuck for what do do next, provide a pick list to jump-start ideas
- remind you that this is how much/little you get to do on your turn to speak
- prevent the GM from getting in a rut in the first place by keeping all their move options visible
The list is super-important, because Dungeon World is not your average dungeon crawling RPG, and not using the list of moves usually results in running it as if it's some other game, and then wondering why it's not as great as everyone keeps saying.
In practice, what goes through my mind is either
- "Okay, my move. I already know which and have a good idea for this one, so let's do it. Let me pause a minute to work out my narration so it's well integrated with events instead of just slapped down disconnected…"
- "Okay, my move. I have no clue! What looks inspiring here, or which have I not used recently…?"
- "Okay, so what happens is… Wait. I haven't consulted my moves lately, I'm probably letting habit run my responses, better check the list quick to be sure this is my best move right now," followed by "Yep!" or "Oh hey, that would be much more interesting/fitting/novel."
Best Answer
Let's start on page 221, under Elements of a Monster. Under HP, it says HP is a measure of how much damage a monster can take before it dies, at which point it flatly dies. Just below that, it describes Special Qualities as "a guide to the fiction, and therefore the moves".
The crux of the issue with the Hydra is it has the Special Quality of "Only killed by a blow to the heart". As such, this overrides the more general rule about HP, and a hydra with 0 hp isn't, in fact, dead.
So, what happens?
You follow the fiction.
This hydra has taken enough stab wounds, magical assaults, and crushing blows to kill a dragon whelp, lich, or chimera. If a head was cut off and didn't regenerate yet, it's likely still gushing blood. It's still limping from that mace blow to the leg. That last magic missile to the heads probably just knocked it out, or it finally succumbed to the blood loss. Or maybe the Ranger got a lucky shot where the Wizard seared a hole in its chest earlier, and an arrow found its heart, killing it. But supposing it was only a flesh wound, the fiction should indicate the way it's out of the fight.
So, what happens next?
You follow the rules.
The Hydra lists "Regenerate a body part (especially a head)" as a Monster Move. Those are a specific kind of DM move. As such, that move is triggered at GM discretion (in lieu of any other DM move that could fit the fiction at the time), and when one of these three things happens:
Or maybe none of those three things happen. Say the whole party decides they've vanquished the hydra, then wanders off. Pretty soon, the hydra will regenerate. If and when it does, the description requires that it come back stronger, and with more heads (if one was cut off in the battle). This is when the DM will follow the principles "make a move that follows", "think dangerous", "think offscreen, too", and "give every monster life". Those in turn help the DM fulfill the Agenda items "portray a fantastic world" and "fill the characters' lives with adventure".