on Jane Friedman site:

I love learning how the brain works during the writing process. I’ve mostly been interested in how to “turn on” the brain’s right, creative hemisphere through exercises like free writing. A book I recently read gave me insight into a different aspect of the writing process: the value of not writing—of setting aside an unfinished draft.

How We Learn by Benedict Carey (Random House, 2014) explains scientific research on how the brain learns. While much of the book concerns rote memorization, I was drawn to chapter 7, which details how the brain approaches long-term projects (such as writing a novel).

Most writers have heard this advice: set aside your draft to revise later. And most of us have experienced new insights when coming back to a manuscript. Why is it helpful to set aside a draft? Why do we sometimes get insights about our writing when we are not working on it? Carey calls this process “percolation.”