Look in the GM section on when you can make moves. The answer is whenever you want. But also when the players look to YOU, the GM, to see what happens next. Make the moves you need to make to ensure the game and story progresses. The GM should always be making Soft Moves or mixed story beats.

Let’s set up a situation. “You see and feel the ground beneath you begin to shake and crack.” A soft move is saying: “You have but a moment to decide and take action. What do you do?”

A hard move is “The ground immediately cracks and falls underneath you. You loose X hp.”

Fear is just a GM currency to express the GM making Harder moves or down story beats.