John Fahey, thought during his lifetime to be possibly more than a little crazy, was the author of some thirty albums of gnomically introverted droning guitar instrumentals, which I listened to heavily in my teens and twenties; I even produced an hour or so of banjo music in an imitative John Fahey style.
Normally, most writers don't say, 'I'm going into a mild hypnotic trance.' Typically, they don't know how they do it. Most people, when they have a good experience writing, they're well placed in that state, which is also sometimes called a 'flow state.' If you don't have trouble, you don't have to think about it.