You're writing along, and suddenly you realize there's a flaw in the logic of your plot. You know where you want them to go, but you don't know how to get them there believably. You stop for a while, think it over, maybe jot some notes. You ask your characters and listen to their answers, but they don't know anymore than you do. Might take you hours or days, but eventually you put that scene aside and pick up the next scene, the one in which you know where they are, if not how they got there, and you write. And then, once you've well and truly released it, just like that your problem is somehow solved. It could be from a magical keystroke jostling your memory, or the water hitting your scalp in the shower, or a song your husband hums while shaving. Whatever it is, your brain makes the connection you were lacking, and the way is cleared. And it gets easier to go on faith every time you reach an impasse. The answer is in your brain somewhere, but you can't manufacture the key. The key is a gift, and you have to wait for it to appear. And when it does, it's a great feeling.