Since this is sitting on the front page, I might as well take a stab at it. I'm never one to miss a chance to misinform someone. First, Nadal. Obviously, Rafa has a unique forehand. Instead of his swing moving away from his opponent on the backswing and toward his opponent on the forward swing, like most bent-armers, Rafa's arm (he's a lefty) moves like a runner whose left leg is striding forward. His elbow moves pretty much from the chest side of his body, right past his hip, to the back near his left buttock. Why? Because, if he reaches back like, say, Djokovic, he couldn't have that compact straight-arm swing. You could get your arm straight, but with slow racquet head speed and hard to make clean contact. (DelPo is doing something completely different and is his own story.) Rafa rotates 90 degrees and then finishes all arm. Swinging hard with just the arm after arresting the torso's rotation promotes a straight arm swing. It helps to go from low to high, and you can see how that is easier with the elbow pretty much straight down below the shoulder. BTW, everything in the last three sentences contributes to him following through over the head like he often does. So, why does he **** the racquet? By pulling the throat of the racquet back, it pulls his elbow further in front so it has longer to travel, giving him a longer backswing i.e. more momentum. The resulting change of direction, from the now more momentous backswing to the forward swing gives him more lag and helps make things more fluid in general.
Stan? I don't see a ton of cocking the wrist like Rafa on the forehand, but he does it a little. It helps so that when he drops the racquet into that "fishtail" position by his left hip, his arm is about as fully pronated as possible. Since the one-handed topspin backhand is supination, it helps to come from fully-pronated for power and spin.