The piano-playing hand doesn’t maintain its natural structure without some muscular energy. I’ve heard fine teachers call this “flexion” but the word that physicians use is “tonus”, the alternating low-level contraction and relaxation of neighboring muscle fibers that holds parts the body in a neutral functioning position without getting tired.
I’m not sure that this is what most piano teachers mean by “relaxation”. Many of my pedagogy students seem to be under the impression that a good technic requires total relaxation at all times and that arm weight alone is enough to play the keyboard.
I disagree. To paraphrase Edna Golandsky, former Artistic Director of the Taubman Institute and now director of her own eponymous festival, movement is impossible from a state of total relaxation. If you don’t believe us, try getting up from a prone position without flexing any muscles. Impossible.
Different dynamic levels and sound qualities require different types and amounts of muscle tonus. To give my students a basic idea of how this works, I use stuffed animals. Yes, even my university students.
Take a smallish stuffed animal and grasp it in your hand. Flex your fingers enough that you can comfortably hold it in your hand, not squishing it, but not relaxing so much that it drops, either.
Look how great your hand position looks. That is, more or less, the degree of muscle tonus we need to play the piano.