A combination of biomechanics and high speed video work together for understanding. A lot of what you see in high speed videos of tennis strokes involves the Stretch Shorten Cycle.
You use the misleading term "unit term". What is the "unit"? The upper body is defined as the body above the waist. Do you mean the body above the waist turns in a unit, like a barn door? But if we look at some of the biomechanics that are seen in high speed videos, we don't see what the words "unit turn" imply. What we see in a high level forehand high speed video is trunk twist. Why? Biomechanically, it's because the muscles of the spine and trunk are being stretched by twisting as part of many Stretch Shorten Cycles of many muscles.
High speed video is the "Mental Picture" that your are after and biomechanics is the rational theory that leads you until your discover that something is not well described. Look at Djokovic when he wants to hit a heavy pace forehand, look at the body between the hips and uppermost body (around the shoulders). You can see this watching a match on TV. But the science of biomechanics has appeared and progressed, not perfect, but the best approach around. Search:
separation forehand Djokovic
The Kinetic Chain Concept (KCC) seems to have it origins with Bunn in the 50s to 70s. There is a widely referenced book for coaches.
The Kinetic Chain Concept states that body parts speed up and 'transfer' ??? their speeds in a sequence to smaller and more
distal body parts, the last of which is the hand or foot. That is somewhat true, sort of. But thinking in terms of speeds misleads because it downplays or has nothing to say about the stretching of muscles and their Stretch Shorten Cycles. In other words, KCC emphasizes the kinetic energy of moving body parts but ignores the potential energy of stretched muscles. Once you see a tennis stroke as sequences of sub-motions to stretch and use many muscles, you can watch a high speed video and see what most likely is going on. If you view a high speed video as 'kinetic chain segments', sequentially speeding up without muscles being stretched, you are missing the stretched muscles that are critical to how tennis strokes work.
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It helps if we are always clear about defined joint motions vs the body part we most see as moving. For example, the shoulder joint vs the shoulder mass. Words are often ambiguous.
Agassi in the OP -
"quit thinking a million different forehand thoughts, take a light grip on the racquet, relax your right arm, and just think of hitting the ball with your right shoulder. Rotate that spot of duct tape into the ball."
That can be a very good piece of advice as long as it's clear what 'shoulder' is and if some additional motions are added. Here Agassi is using "shoulder" to mean the part of the body, the shoulder mass, seen and not the shoulder joint. In the actual stroke, the uppermost body is accelerated to turn rapidly - and move the shoulder mass with the tape on it, but also the shoulder joint moves the upper arm before impact. Simply looking at videos can show you what is going on. The video has much more complete and accurate information than the word description. But that word description adds "light grip" and "relax" not in videos .........
Djokovic forehand showing separation on the take back, hip lead on the forward swing. Separation - imagine a line between the shoulders, imagine a line between the hips, imagine how those lines would appear if viewed from above, that's the separation angle that indicates the stretch of twisting trunk muscles.
Single frame on Youtube use the period/comma keys.
