When after years of piano lessons I finally found my True Teacher, Robert Sheldon, we spent an hour on this concept and I played exactly one note: D.
Mr. Sheldon was not talking about dieting. His point was that it is the speed with which the piano key descends which produces loudness or softness. If the key goes down slowly, the resulting sound is soft. If rapidly, the sound is loud.
Most piano teachers teach the way they were taught, and Mr. Sheldon was carrying on the lessons of his own teacher, the great Egon Petri (see http://www.pianoeu.com/petri.html.) Weight on the keys is largely expended after the key has reached bottom or keybed. Petri and Sheldon spoke of "taking" the key, rather than pushing or striking the key. Applying weight or force to the key produces very little besides a percussive sound.
What this means for you is that you use a grasping motion on the keys. This may sound simple, but it is so different from most piano methods that a description of it on line put me in touch with a teacher in Finland who proved to have the same second-generation Petri lessons I had.
Your position at the piano should be comfortable and far enough away from the keyboard that you have a straight line from your elbow to the knuckles of your hand. You need to adjust this position to accommodate the sheet music you are looking at, your own height, and the height of the bench you are using. The part of the finger which takes the key is the fleshy pad where your fingerprint resides, not the tips of your fingers.
Vocabulary, Lessons Three and Four
Interval: The distance between notes or keys
Do-re-me-fa-so: Note names used in some cultures. In "fixed do", Do is always C. In "Movable Do", the tonic or beginning of any scale is called Do.
Percussive: A struck sound, like a drum.
Keybed: The wooden platform below each piano key.
