Wikipedia defines psychological flow this way:
Flow is the mental state of operation in which a person in an activity is fully immersed in a feeling of energized focus, full involvement, and success in the process of the activity. Proposed by Mihály Csíkszentmihályi, the positive psychology concept has been widely referenced across a variety of fields.You've probably heard about people who claim that the writing just flowed (and you've probably felt a bit jealous of them). It's hard to hear such a thing without, 1) taking it to be something mystical, and 2) judged yourself to be a lesser writer for not being able to make a similar claim.
I can't guarantee that you'll always enjoy flow in your writing, but if you understand the nature of the state then you might be more likely to experience it.
The most important thing to understand is that there's nothing mystical about flow. Indeed, it is effectively the opposite of mysticism because you're neither awed nor terrified. When you're fully immersed in the process you find, to the extent that you're even aware of your internal state, that you feel a profound calm.
Flow is like Baby Bear: you're neither too hot with great ideas, nor too cold bogged down in the details, but just right with the ideas and the words to express them coming together at the same time.
I've heard people argue that writing is a purely creative, right-brain activity. There's truth in that claim, particularly for those who see the action and the setting, and hear the voices of their characters. But encoding those ideas in well chosen words and ordering those words in compelling, grammatically correct sentences is a left-brain activity. Of course, your inner editor lives in your left brain.
People who focus on one side or the other short-change themselves. In my experience, flow is most likely to occur when I've mastered the left-brain mechanics (i.e., proficiency at typing, a command of grammar rules, a rich vocabulary) and energized the right-brain to focus on the story (and not entertain every distraction that comes along). Put another way, you can think of flow in writing as balancing right and left brains to produce and encode ideas.
Deren blogs daily at The Laws of Making.