On "knowing when to come in" the nod does work but especially with good eye contact communication. Sometimes I have thought that this is the "real" language of music, when just looking at someone eye to eye communicates a thought process that makes spoken words completely unnecessary. It's quite a beautiful thing and I believe it is what separates musicians into who are and who are not true group players.

In bluegrass, it can be on levels the novice can only imagine.