It is important to distinguish descriptions of how piano playing feels to the pianist, from what a pianist is actually doing. Even great pianists' descriptions of what they are doing may be grossly inaccurate from a scientific point of view. For example, often one reads about aiming for "complete relaxation". As a scientific description, this is nonsense: Complete muscular relaxation only happens after death! Piano playing requires muscle tone, and even muscular effort. What is being described here is how a good pianist looks and feels: uninhibited, without excess tension.