You're wading around in your notes, getting nowhere. You don't see a structure for the piece you're trying to write. You don't know what to do. So you stop everything and hunt through your mind for a good beginning, a good way to scissor in. Then do it; write it; get a lead on paper.

If the whole piece of writing is not a long one, you may plunge right on and out the other side and have a finished draft before you know it. But if the piece is to have some combination of substance, complexity and structural juxtaposition,...