I find this a fascinating topic, yet hard to describe; for me "groove" happens in an almost spiritual space that exists outside the words used to describe it. I think it's pretty close to the idea of "flow" in psychology, though don't really know anything about that specifically (http://en.wikipedia.org/wiki/Flow_%28psychology%29). To me this is one of the experiences that makes old-time really special for me and worth slogging to a session far away.
Groove for me is when you're playing like crazy, heads leaning in together, and the music takes on a wonderful quality almost like it's playing itself, where even really fast and challenging tunes become almost effortless, yet beautiful. Sometimes there is drool :-). When the tune ends everybody gazes around at each other, going "whoa.... wow...". That ever happen to you?
I sometimes call this quality 'hydroplaning' - because of the way the tune will "lift" and start moving easily and freely. Like it's skimming above all the effort, like a skeeter will shoot across water without have to wade through it. improvisation becomes easier too, and sometimes I'll find that my improvizations - say playing up the neck, or a few harmony notes or even dissonances - will coincidently be perfectly in sync with the other player(s).
There was an interesting bit of research a couple years ago I think, where they wired up some musicians and measured their brain waves while playing together. Their brain waves actually synced up and became more similar to each other (http://www.scientificamerican.com/blog/post.cfm?id=musicians-brains-keep-time--with-on-2009-03-16). My first thoughts reading the article were "that explains a LOT", and "they really should have used old-time musicians!"
My guess is that old-time is very suited to this kind of experience, because taking 'breaks' are less common, the relatively simple yet rhythmic tunes are repeated over and over, and there is a real effort to play WITH the other people (or, I hope there is!). i.e., more than many other kinds of music, in old-time groove is a *goal* of playing together, whether you know it or not.
Things that hamper "groove" for me are tunes in a style I'm not familiar with, way out of tune, and - most frustrating - other musicians who will play the tune perfectly yet aren't willing to listen to or 'sync up' with the other players. Among some musicians I know there are a couple *amazing* fiddlers - but I don't enjoy playing tunes with them as much because I feel like they are in their own bubble. They may be playing beautifully, but they aren't playing WITH me. And there are some other, intermediate fiddlers who I have a great time with, because I can feel like we are playing *together* - we can find a groove.
Anyone else experience this, like this? Differently? What are the circumstances that help it happen? Has it never happened? Never thought about it? Do you find that being more skilled helps more? (I do find that the less I need to think about the instrument the more it can all just 'flow'). Is there something that you do to increase the experience?
Perhaps this should be a blog post instead? Maybe I'll copy it into one and start my first entry... :-)