The serve is similar to the forehand in its mechanics.
I would disagree with this pretty strongly, because a FH is not a shoulder-over-shoulder motion. The cartwheel motion means that you have a pivot motion around the hip that is unlike anything else in tennis. Different set of muscles with the kinetic chain (shouldn't feel the biceps or thumb during pronation.) Different stance. Wrist release through the contact point.
A lot of beginners try to build their serve with elements derived from their FH groundstroke, and that is why it's advisable to shadow serve using a sock.
In fact, the modern forehand has been described as "a sidearm serve."
Would also disagree with this. A shoulder over shoulder motion tends to put the arm in a 3/4 slot. Elliott amended his position in a later article,
The forehand specifically relies on the pectorals, deltoids and biceps to provide much of the upper body and arm activity in a tennis stroke, with the forearm and wrist "following along for the ride" after the hips open and generate internal shoulder rotation.
This is true: pectorals, front deltoids, and biceps. And remember that the serve emphasizes different muscles: pectorals, medial deltoids, and triceps.
You can tell if the shoulder is included in the stroke by how the elbow displaces. A high elbow finish equates shoulder rotation, low elbow finish equates no shoulder rotation but all wrist deviation.
Ehh, I actually have to disagree with that. It's like reading a racquet drop, a lot of factors go into the elbow finish, and if you know what to look for, you can make some educated guesses. For example, if you look at the angle of their forearm relative to the ground, you can guess how early their pronation was executed in the serve. If you look at the overall height of the upper arm against the shoulder, you can surmise how much "angle" (i.e. transversal adduction) they got in the service motion. It is however more useful (and easier) to evaluate a person's windup around the point they are in the trophy pose.
after the hips open and generate internal shoulder rotation.
Yeah, hip rotation loads shoulder rotation. However, hip rotation isn't the same thing as winding up the hips. Using the one foot drill helps you learn to use hip rotation in your strokes and your serves.
when done this way, you are generating tons of internal shoulder rotational power not gotten from a more wristy shot. this is completely different from "finish as if you are trying to tell the time" which might or might not amplify the power from ISR.
Actually, this isn't really true. Both a WW finish and an over-the-shoulder finish produce a lot of shoulder rotation. A lot of people associate turning the shoulder back as loading the shoulder, and actually this isn't quite true. Again, shoulder rotation really is about hip rotation, and that in turn reflects a person's overall footwork. Using a step-out pattern helps a lot with this. The one foot drill helps with this.
Your momentum and balance is important with the grounfstrokes. It is what your body uses to have the muscle stretch and load the kinetic chain. In terms of the video clip, the left arm is really a guide for that momentum. Ideally, as your momentum goes forward, you set up your left arm as a guide for your shot. Then when you initiate the forward swing, the left arm reacts as a brake to keep your torso aligned through the shot. The one foot drill enables you to learn the form this way, so all the dynamics are integrated as one.