From what I understand, during direct examination:

- It is assumed that the witness you called is favorable to your case, so you cannot ask them leading questions (in order to avoid coaxing them into giving the answers you want).

- However, if the witness ends up being *not* favorable to your case, then you can have the judge declare him to be a "hostile" witness, therefore allowing you to ask leading questions.

But then, what in the world is the point of prohibiting leading questions by default?  
If they're giving you favorable answers already, then you don't need to ask them leading questions. 
And if they're not, then you *can* ask them leading questions anyway, so what was the point of prohibiting that in the first place? I don't really understand the logic.