Music Composers Unite!
for guitar, its all about facial expressions! :D
I believe its the ability to be inside of, and actually be part of the music, as opposed to being outside of it, and simply performing or playing the notes..
A kind of merging and being one with the music, and letting it play you...
I definitely subscribe to Bobs comment as I've had that feeling many times when performing. Hard to describe, but it feels like you're not actually you doing it, more like your channelling energy or a force. Sort of an auto-pilot sensation where your normal characteristics and mannerisms have been put in a limbo or trance-like state allowing this connection to happen. I think it has alot to do with the intent of the musician, their state of mind and also how the piece affects them. A very interesting subject!