When you get in that groove in which you can't stop. You may have to take a break for a bit because your brain fatigues; but even if that break lasts an hour or two, your work draws you back. You have to tweak a quote, find another, shift that sentence there because it doesn't quite say what you want it to say. You may not add many more pages, or carry the story forward very far, but you coax it, smooth it, make it better until you reach a point at which you know you are, in fact, writing crap and that you will spend more time tomorrow fixing it than you will if you just stop and pick up again. You look up, and realize how late the hour has grown, then you collapse into bed to read a novel, and fall asleep holding the book open.
That's what's cool.