Messy weapon vs Barbarian Smash move clarification

Question

Sunday I had my first Dungeon World experience as GM, with a group of people completely new to DW too. During the first fight (sadly the only thing we manage to do since we had less time than usual and we were slower than I expected) the barbarian charged the leader of the enemy team. When he got to the enemy he tried to chop his arm off. I let him do it, since his weapon was Messy, with the "hack and slash" roll which he failed.

Today, thinking back to that scene and reading again the barbarian move, I have some confusion.
The barbarian was level 1, so he hadn't the Smash! move, which allow him to easily cut off the arm.
On the other hand his weapon (like any weapon used by a barbarian) was a Messy weapon which allow him to "Do damage in a particularly destructive way, ripping people and things apart".
I don't see any use to the Smash! move since I can always say I'm going for certain body part, deal my damage and remove the part thanks to my weapon always being Messy *.
The problem repeat with Forceful too.

Am I missing something about Smash! or am I giving too much credit to Messy weapon?
How the Smash! move add something to the Barbarian?

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* Not that I suggest someone should act always like that, it's an example.

Answer 1

DM vs Player discretion

Messy and Forceful being weapon tags means that ultimately how and when they are invoked is usually up to you as the GM. It is totally within your power to describe him using his weapon in a way which might be particularly gruesome due to the nature of the weapon (think about wounds from large swords or hammers). However just because the tag is there does not mean that you need to always have it actively influence how you adjudicate/narrate what is going on as the GM. I think in the example you gave I would not have it lop off, but rather only if it was the killing blow.

Smash! being a class move and in its description stating:

"deal your damage and choose something physical your target has (a weapon, their position, a limb): they lose it."

Means the player picks what they do. They get to destroy a specific limb or break a specific item. Maybe they want to cripple the leg so they can't be followed or break their enemy's weapon, its totally up to the player.