A Druid shapeshifts into a monster. They spend their hold on monster moves.
There's some further discussion of this here, as it relates specifically to turning a pre-existing list of monster moves into acceptable things for the Druid to spend hold on. But let's start at the beginning. When the GM is writing a monster, what do they write down as monster moves? They don't write down that the monster can:
- breathe water
- see in the dark
They write down the cool plot things that those abilities let the monster do:
- carry something precious into the sky
- escape beneath the waves
- assault them from the darkness
The Druid still has all the new form's basic properties and weaknesses, which can change how appropriate certain moves become - a mouse probably has no way to Hack And Slash at an ogre, a fire elemental can drift over a pool of lava no problem, a treant can bar the way more easily but is also more concerned by fire. The boundary between "spend hold to do it" and "just do it, don't spend hold" is the point at which taking advantage of the new form's capabilities becomes something cool and exceptional.
This isn't a new list of player-facing moves. You know, the kind that have you roll dice and make choices? The Druid's a monster, they get hold for monster moves, and when they spend them, they act like a monster - the cool plot thing just straight-up happens. The player should know roughly what impact that's going to have - tell them the requirements or consequences, right?
It's possible that the Druid may have to make a player move, such as Defy Danger, in order to get into position to use one of the monster moves, since they don't have the GM ability to deploy themselves in whatever position is the coolest. The move may also have aftereffects, like rolling damage or making choices, that would be similar to the way player moves shook out if the dice landed a certain way.
And this isn't a contract signed in advance. When the shapeshift lands, the GM should have a couple ideas on tap -- and of course, the Druid probably had their own ideas as well, or else why did they even pick this form in the first place?
But the GM always has a couple things in the toolbox: show an opportunity that fits a class's abilities. Tell them the requirements or consequences and then ask. So if Leafwillow turns into an eagle with a wooden leg in the middle of combat, the initial suggestions from both the GM and Leafwillow are probably going to relate to combat. But if the fight closes out and Leafwillow still has hold left, and the party has some ground to cover, the GM is perfectly fine to say that Leafwillow can use that last hold to run overwatch until everybody makes camp for the night or finds something suitably diverting.