In the continuing argument I'm having with some friends about piano playing, I'd like to explain a little more about my position. Here is a description of the piano action from www.teach-me-piano.com.
What Happens When You Press A Key
The key (1) front goes down, the key back goes up, pushing on the capstan (2), which pushes the wippen (3), which pushes the jack (5), which pushes on the hammer knuckle (circle under the hammer shank (8)).
When the hammer is half way to the string, the back of the key begins to lift the damper lever (12).
When the hammer is almost at the string, the jack toe hits the letoff button (4) (regulating screw), and the jack slips out from under the knuckle, freeing the hammer to continue on its own inertia.
The hammer hits the string, and rebounds. The knuckle lands on the repetition lever(9), pushing down the repetition lever.
The hammer tail catches on the back check (11), and remains there until the key is released.
Here is a great link for an animated image of the workings of the piano mechanism.
The argument that I am making is that the pianist has control over the velocity of the key drop. To the best of my knowledge, we are talking about a 1cm descent. The velocity of the key depression determines the speed of the hammer throw. The speed of the hammer throw determines the volume and timbre of the string vibration - the timbre being determined primarily by the quality of the instrument.
The argument that many pianist make is that volume and timbre can be determined independently by means of the angle and quality of the key depression. The most common argument that I hear is that by drawing the finger back toward the body when the key drop happens produces a "warmer" tone independent of volume. There are many variations including arguments about angle of attack and brush strokes and all other sorts of imagery.
The argument is basically this: Your "warm" tone is being communicated through the key drop into the capstan which pushes the warmness into the wippen which then makes the jack feel warm and fuzzy, so it gently nudges the hammer shank. The hammer shank is so tingly that is rises up and envelopes the string in its felt in a more gentle way than it would have if your finger had come straight down on the key.
One dear friend even made the argument from authority and said, "I bet Arthur Rubinstein, Alicia de Larrocha, Claudio Arrau, Rosina Levhine, Ignaz Friedman, Cortot, Gilels, Sokolov, and many others would feel terrible. They were wrong the whole time!"
I wouldn't suggest that I can play as well as many others - especially that formidable list, but if they thought that they were changing the tone of the instrument by the manner of their key stroke, they were indeed wrong. It's not physically possible. What is actually happening is that the control of the velocity of the key stroke produces and emotional response that is then being perceived as a different tone quality. That's all good. You should play emotionally. You just don't need to introduce extraneous motion in the key stroke in order to accomplish it.
A final question came from another friend. Does change in acceleration become a factor? The answer is: no. Because of the escapement action, a certain degree of force is necessary to throw the hammer, but the hammer is being thrown by the wippet. Once the wippet engages, it is not possible to adjust the acceleration via the playing mechanism of the key.
This also means that all those reviews you read that say this or that pianist had such a great "touch" are nonsense. There isn't such a thing as "touch" if the laws of physics remain constant. There is only the velocity of the key stroke.
So, play emotionally. Use imagery. Do all that stuff. Just don't waste time trying to do motions that don't affect the outcome. Efficiency in physical motion is the basis for virtuosity.