A comment from a fellow musician got me thinking more literally about the analogy from "getting in the groove" musically and in life in general.
Being in the groove is having expectations matched to circumstances, so you don't have to give them much attention, which frees your attention to stretch yourself within the groove. We get "rattled" or "jerked around" when expectations aren't met, because unmet expectations forces a dilemma, "should I change my expectations here, insist that they get met here, or go somewhere else?" Doubt about which of these to do is taxing.
A groove is what Alan Watt's was talking about when he said the trick is to "understand the patterns structures and trends of human and natural affairs so well that one can spend the least amount of energy dealing with them." A groove frees your energy to explore, stretch and grow within the groove.
A groove is freedom through safety. It's being able to try unsafe things within an overarching context that holds you safety. It's got the qualities Gregory Bateson noticed about play, for example dogs' play fighting. It has to feel real, intense and on the edge, but not so much so that it's dangerous. It's room to push the boundaries because the boundaries contain us so well.