While working on the current novel, something stuck in my head.
I wouldn't say I'm a pure "fast writer", but I do write fast a lot of the time. I also outline, make notes about the story, and do some pre planning so it's not just me fighting the white bull to a capacity crowd of nobody.
But once the story starts up, I basically drive it forward to see where it goes, remember the cool bits from the outline but never being shy of letting a better idea take its place. And in the past two books, cool stuff has come up on the fly. Characters, villains, incidents, turns of plot.
And it reminded me of improv comedy. I never formally studied improv, but I've spent a lot of my waking life telling jokes and stories, going into character, putting on accents, and trying to make people laugh. Generating comedy material from all the comedy shows, sketches, movies, stand up I've experienced, plus all the other weird stuff in my brain. And doing it on the fly.
I think most improv is based on talent, experience, results, and an ability to develop a gag out of the junk in your head (pop culture, history, acting skills, familiar scenarios, etc). Maybe fast writing taps the same system?