The underlying purpose of the mechanic is to make the characters behave in emotional ways, no matter what. The first two moves (Questions/Bargains) are for lower stakes situations, while Fostering and Threatening are for dramatically important situations. Normally, you expect that the characters will…

Deal with the challenge procedurally, typically by using their Professional Moves which don’t involve dice rolls. They behave as hyper-competent badasses.

Express emotions, even if they are prideful confidence, by narrating character reactions. They behave as emotional creatures. It doesn’t matter what the emotion is, just that there is an emotional reaction.

In most dramatic situations, the players need to do both. If they fail to act procedurally, they will often suffer Harm. If they fail to act emotionally, they suffer Stress to a relevant track.

Example: A corporate agent is offering Whistler a job, continually putting more and more fine scotch in his glass. If Whistler negotiates with him but refuses the scotch (which would hinder his negotiation), his Need would increase as he represses his desires. If he gets plastered on the scotch and neglects to negotiate effectively, he goons will rough him up (harm), kidnap him (seize control), or get some other advantage.

As a player, you want to set up a scene where your character indulges in his desires, negotiates despite the obvious bribery, and stumbles out of the room with the job in hand. /example

There are many smaller situations where only Foster or Threaten make sense, but I figure the crux of your question was on the more complex cases.