Those who run frequently have experienced the flow of endorphins which kick in after a couple of miles and makes you feel like you are being propelled forward. You are in the zone.
It's the same thing for authors, and I assume, artists. You don't know when your characters take over the story and tell you what they should be doing, but when it happens it's almost like your writing is being driven by an exceptional source. You don't want to stop, because it seems so perfect.
I'm not saying that everything you write when you're in the zone will look great to you upon second reading, but while you were writing it, you were being pushed forward by unseen winds.
When I look at some of the art that's been produced by people who are on my Google plus, I'm astounded by the beauty. Maude McDonald had the creativity to paint a modern horse against the back ground of the figurative horses in European cave paintings. It's clear she was in the zone when the idea struck her and she painted this work of art.
Unfortunately, at least for this writer, some days you sit in front of the computer, and until you try free writing you don't have a chance in the world of entering the zone. That's why it's such a special place, because the rest of the time you are struggling to create a word picture out of the ether.