The next time you watch a great single-handed backhand, watch what the player does with their non-racket arm.
The other arm plays a far more important role in the shot than many people realise. It plays a supporting role at the throat of the racket during the grip change and take back, right up into the fully loaded position. It then plays a starring role as the racket starts it’s follow-through.
The non-racket arm is either floated back or sometimes accelerated back (depending on how hard you are hitting) to a counter-balanced position. The use of this arm as a form of balance is fairly obvious and if you put your left hand in your left pocket (right-handers) and keep it there for the duration of your shot, you’ll soon see what I mean.
The non-racket arm is utilized in the same way for a slice backhand, though it finishes higher for the slice and lower for the drive (see Tim Henman’s photo with his left hand lower).
The other reasons for separating the arms are that this action slightly sqeezes the shoulder blades together which aids the generation of extra power in the shot, and also it helps prevent the chest rotating round to face the net too much.
Tim Henman has used this technique with great success over the years.