Ebbie: "one uses a different part of the brain" .. I have to wait for that to happen before I can write. I always call it a "Brain switch". During the writing process, words flow with ease, as do the tears. By the end of a good piece, I am an emotional wreck and in some cases have stopped writing, just to give myself a break.
Therapists will often suggest that you write about your emotions with your non dominant hand in order to trigger this Brain Switch.