Engagement in a challenging activity that requires skill. Altered sense of duration of time, or sense of being taken out of time. I have surveyed some of my classes and Writing Center staff members about their optimal writing experiences. Their replies seem to jibe with the elements of flow. Here is what some of them reported.
Ideas come freely.
Transitions seem natural. It was like the pieces of a jigsaw suddenly fell into place. I saw connections I'd never noticed. I almost couldn't type fast enough to get all my thoughts down before I lost them. The words were like notes of music that I could hear in my head.