First of all, you don't use moves in DW. There are three general flavors of moves that change how the game works for your character.

1. The most common type; triggered moves. These come into effect when you describe doing something that matches the trigger description. Hack and slash is the typical example.
2. More on the advanced side, the modifier moves. These enhance or alter how other moves (basic or class) work. Usually they give you more or better choices, or add a 12+ tier. They come into play through the move they modify.
3. The often underestimated permission moves. These allow you to access otherwise unavailable rules, moves or effects. Cast a spell is a typical example, and the multiclass moves are of this category, giving you permission to pick moves from other playbooks.

Now for the core of your question; since the Multiclass dabbler/initiate/master is a permission, it has no trigger. It does not manifest when you describe taking an action. It just allows you to do what it says in its text. And when you read it carefully, you will notice that it says

Get one move from another class. …

(Emphasis mine)

It says "one move". Not more. It says "get". Not "access" or "use". It becomes one of your advanced moves like any other. That's it.

And there's another thing to be aware of in Dungeon World. "Making a call" or "making house rules" is not a part of the DM's principles, agenda or moves. Contrary to the tradition in many older RPG's including D&D, what the DM can do is pretty much prescribed and structured in DW. It is an integral part of how the game plays and those rules are not optional recommendations. The game does break if the DM fails to observe them. Caveat dominus tenebrae!

First of all, you don't use moves in DW. The moves affect play per their description. There are three general flavors of moves that change how the game works for your character.

1. The most common type; triggered moves. These come into effect when you describe doing something that matches the trigger description. Hack and slash is the typical example.
2. More on the advanced side, the modifier moves. These enhance or alter how other moves (basic or class) work. Usually they give you more or better choices, or add a 12+ tier. They come into play through the move they modify.
3. The often underestimated permission moves. These allow you to access otherwise unavailable rules, moves or effects. Cast a spell is a typical example, and the multiclass moves are of this category, giving you permission to pick moves from other playbooks.

Now for the core of your question; since the Multiclass dabbler/initiate/master is a permission, it has no trigger. It does not manifest when you describe taking an action. It just allows you to do what it says in its text. And when you read it carefully, you will notice that it says

Get one move from another class. …

(Emphasis mine)

It says "one move". Not more. It says "get". Not "access" or "use". It becomes one of your advanced moves like any other. That's it.

And there's another thing to be aware of in Dungeon World. "Making a call" or "making house rules" is not a part of the DM's principles, agenda or moves. Contrary to the tradition in many older RPG's including D&D, what the DM can do is pretty much prescribed and structured in DW. It is an integral part of how the game plays and those rules are not optional recommendations. The game does break if the DM fails to observe them. Caveat dominus tenebrae!