Have you ever drawn an entire picture, or painted a scene, to step back and realize that you've captured details that you didn't even notice were there while you were drawing it?
This is what I've heard referred to as a creative, or artistic zen.
You're capturing a landscape or a bustling city scene, and it has become almost muscle memory. You're no longer consciously looking at your reference for details, or at least not to the same extent you were years ago.
Many think that it takes years of practice, grinding into hundreds of sketchbooks.
More than once have I come across artists who draw rarely, but seem to improve in leaps and bound