Here are some theories I found when I googled "hands behind back."
hands joined behind your back – this stance (the “parade rest”) makes
you seem like you have no energy
http://marjoriebrody.com/showArticle.asp?id=1 Body language is also key. I’m inclined to slouch on one hip and I instinctively put my hands behind my back: advertisements that you are feeling nervous, apparently.
http://www.skillstudio.co.uk/red-1.htmAnd, here is how George Orwell interpreted the gesture in 1984:
The man sat down on the bench at a little distance from Winston. Winston did not look at him again, but the tormented, skull-like face was as vivid in his mind as though it had been straight in front of his eyes. Suddenly he realized what was the matter. The man was dying of starvation. The same thought seemed to occur almost simultaneously to everyone in the cell. There was a very faint stirring all the way round the bench. The eyes of the chinless man kept flitting towards the skull-faced man, then turning guiltily away, then being dragged back by an irresistible attraction. Presently he began to fidget on his seat. At last he stood up, waddled clumsily across the cell, dug down into the pocket of his overalls, and, with an abashed air, held out a grimy piece of bread to the skull-faced man.
There was a furious, deafening roar from the telescreen. The chinless man jumped in his tracks. The skull-faced man had quickly thrust his hands behind his back, as though demonstrating to all the world that he refused the gift.
http://www.online-literature.com/orwell/1984/18/I'd like to hear from an expert. I've often seen lawyers make this gesture when standing before a judge. It may suggest that the person does not want to show emotion, or feels respect for the other person or is being very guarded about what they say. Maybe Rumsfeld wanted to signal that he was not ready to shake hands, salute or make some other such signal to the person he was talking to. Putting your hands behing your back brakes any forward movement, so it could be interpreted as expressing a desire to hold back or stay pat, one or the other. Interesting.