I don't think nadal can swing only with arm, he would break his shoulder after the first couple rallies if he did that when he was still starting out playing tennis.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.
Only the finish of his stroke is all arm. It is like a whip. Starts the forward swing with torso rotation, which gets the arm moving forward. If he continues with the torso rotation, his arm and racquet would be closer to the speed at which he could rotate his body. Only by stopping the rotation can he swing the arm with high speed. Same principle when throwing a ball, swinging a bat or throwing a punch. Watch him in slow mo. His upper body rotates only to chest parallel to baseline as opposed to a bent-armor like Djoker who follows through with hitting arm shoulder. Two different strokes.I don't think nadal can swing only with arm, he would break his shoulder after the first couple rallies if he did that when he was still starting out playing tennis.