Thanks MGM-Lion, that's exactly what I meant. It applies equally well to gentle lullabies as to lusty sea shanties.

Opening the mouth allows more volume without effort and relaxes at least some of the throat muscles.

I've no idea what "sing as if you don't need the money" means. It just feels right to me. I think there is an implication of singing without having anything to gain or to prove. "Singing to the back" implies being aware of the needs of the whole audience. "Singing without needing the money" signifies, to me, releasing the song, for its own sake. Diction and breathing and expression are what the singer owes to the song.

All pretty much the opposite of what Rahere thought I was saying.