This requires some anatomical awareness, and I'm somewhat "disappointed" with all those who just ignore muscle talk, because that's the foundation and explanation for all this "feeling" and "intuitive and natural things" etc.
Holy cow, it seems like "muscle talk" is mostly what happens regarding technical matters on this forum!
My point is that after a certain point of basic understanding about muscles and body mechanics in playing piano, which is rightly the guide to what one should do, it serves no purpose to go a great deal further due to the fact, IMHO, that one reaches a point of diminishing returns.
Personally, I'm certainly no master of anatomy, but in my pursuit of understanding, I've watched dissections of the hand and forearm, read bits and pieces of Ortmanns' massive scientific work, parts of Riddle of the Pianist's Finger, by (I can't remember his name), and Pianists and Their Technique (Gherig?). In addition, I've read all sorts of miscellaneous articles about the subject of piano playing muscles, technique and so on by both famous and not so famous pianists. Conversations with a number of teachers and pianists in addition to my own ideas and conclusions.
I wish I had a nickel for every hour I've spent over the years on this topic. This is not a particularly impressive list of efforts, IMO, but one that many make, and a good deal of it was "over my head" in that it entailed a good deal of various sciences. The only reason I mention it is to say that I've spent a great deal of time thinking about piano playing physics, muscles and so on. Way too much, IMO.
When people discuss this issue, they tend, by necessity, to talk of one muscle or finger at a time putting emphasis on that particular topic. But IMO, they CANNOT effectively talk about what really happens, which is a subtle contribution of MANY muscles contemporaneously, depending on what type of movement one is making. IOW, it is simply not possible.
What really brought this home for me was watching the arm hand dissection on Youtube.
As the surgeon goes through the forearm pushing aside tissues to reveal various muscles and nerves and tendons as they make their way into the hand through the wrist, what becomes obvious is the sheer complexity of these elements and their interconnectivity and reliance on one another. As one sees where the tendons connect to the fingers, sometimes wrapping around the digits, as another tendon passes through on its way to a different joint, it becomes clear that playing piano is a complex symphonic fugue of muscle movement, not a mere solo or duet, as it often seems when reading explanations.
Add to this that, as I understand it, you might use 5 muscles at once, but 1 or 2 which have more dominant involvement, depending on what you're doing. IOW, again with my very primitive understanding of the topic, not all of the muscle cells in a particular muscle may "fire", depending on the biochemical signal sent from the brain based on the task one is trying to accomplish.
IOW, after a basic point of departure, it is NOT POSSIBLE to understand what is happening, especially given that a good deal of it is not perceptible to the thinking mind.
I agree with you about the forearm tension problem. The instant one puts weight on the forearms into the keys, the large tendons of the forearm automatically begin a grasping movement in reflexive response to keep your arms from falling. This in turn inhibits and tenses the playing muscles in the hand since the large forearm tendons ending in the finger tips tend to act "as one".
Ironically, for many years, teachers taught the "Arm Weight Transference" technique.
IMO, this was (is) the single most destructive piece of "technical advice" ever given.
Attempting to do this will cause tension by supporting this weight of 20 lbs or so, which is insane, especially since we all know (or I think we all know) it is the speed of the hammer into the string that makes the piano's sound and nothing else. Hence, it is the quickness of the key transmitted into the hammer that makes the sound.
This is why, IMO, so much injury occurs in piano, i.e. excessive force, and the unwitting and reflexive use of incorrect muscles due to crazy notions of how to produce sound with their inherently horrible mechanics.
Of course the Arm Weight Acolytes can never explain how slight of stature Asian girls perhaps 100 lbs with delicate arms and hands, can play the Chopin Etudes to beat the band, appearing to "fly over the keys" as they do so.
My Chinese friend I mentioned in another post who played the HammerKlavier quite well, has hands that look like a child's, though she's nearly 30 yrs old. She can just reach an octave, and her fingers and hands are delicate. But play she can with amazing results.
So I hope you gather from my somewhat rambling conversation that I'm not at all anti-muscle talk, its just that one runs into a dead end of unfathomable complexity which would likely cause even hand surgeons and anatomists to pause in lengthy thought were they to attempt to explain the relatively simple piano task of playing a C Major scale.