The modern forehand is initiated through the arms twisting the torso backwards in a big coil, holding that coil, and then working together to initiate a burst of angular momentum in the torso.
Acaususo uses both arms to wind up his torso into a giant coil. Notice how his shoulders are rotated farther than his waist, creating the coil. His arms are holding in the coil, until he is ready to uncoil into the ball.
As he initiates the stroke, focus on his hands in the animation to the right. It's almost as if there is an imaginary rope tying his hands together. Notice how the left hand and right hand lower together at the same time and then uncoil together.
Also notice how Acasuso's left arm sweeps across his body, in order to pull the torso around. His left arm sweeps across, and then bends at the elbow, the same way a figure skater pulls in their arms to increase their rotation.
At the end of the animation, notice how his arms are still in harmony. As his left elbow pulls back, his right arm is coming around at the same time. His arms and hands work together to rotate the torso around to contact.
In the Murray animation, watch how he begins with his left arm across his body. This causes his shoulders to rotate farther than his torso, creating a perfect coil. He begins the stroke by sweeping his left arm around, which initates the torso rotation. And like Acasuso his left arm bends at the elbow and tucks into his side to speed up the rotation and pull the "hitting side" around to contact.
The Monkey Drum
The breakthrough I had in understanding how the arms are used in the modern forehand came from an online instructor named Tae, and his site, Lock and Roll Tennis. Tae uses a simple toy to demonstrate the physics of how the arms and torso create a natural, fluid, coiling and uncoiling motion.
The toy is a "monkey drum". You've probably seen one before. You grab the handle, and as you twist it back and forth, the "arms" of the drum come flying around. by rotating the handle back and forth, the arms get whipped around and bang on the drum in both directions.
Lock and Roll
"Tie" the Hands Together
The final animation in this article is of Alejandra Falla - a great lefty from Colombia. Watch how his hands and arms work together in perfect unison. First they go up together and then back together, in perfect unison. Again, imagine there is a rope tying the hands together. As one moves up, the other must follow.