Hi,
Today my student asked me why pressing a key and then relaxing hand (wrist upward) has the rounded and good sound unlike pressing the key and stiff hand which has harsh and ugly sound.
Could you please explain to me in term of piano mechanism?
Thanks

I once saw a demo, using a pencil, where the teacher could make all kinds of qualities of sound with that pencil. I just tried it myself. I was also able to make a harsh and ugly sound with the "relaxing upward", by landing with great speed on the key which translates into force. So is the thing that you want to have explained actually a reality? What I understand is that if you land and keep pressing without any kind of release, it stiffens the hand and arm, which makes fluid playing difficult, and can lead to discomfort and possible injury.

A pianist's finger controls only two things: the volume produced and the length the note is allowed to sound. There is nothing else although an awful lot of people who really should know better imply otherwise.

A loud FF staccato may sound ugly compared to a P that is held until it dies away, but that is a psychological reaction.

Ugliness of sound only becomes meaningful when two or more notes are played. Then we get into the realm of voicing which is another topic.

It was pointed out to me privately that my earlier post sounds like I trying things out and wondering. I was trying to be diplomatic. A loud sound (which some people might consider ugly) is produced by how fast a key is made to descend. It makes the hammer fly at the strings with more force. I was guessing that the OP was referring to loud, but I agree with Waxwing's explanation.

After the key is pressed, it doesn't matter what you do with your hand, it doesn't affect the sound. My guess is that if your student knows that they are going to have a relaxed hand or stiff hand after pressing the key, this subconsciously influences the way they press the key in the first place, perhaps i.e. they are actually pressing the key with a relaxed or stiff hand, thereby pressing it faster or slower or similar.

I did not comment. The idea that any teacher could possible think that we can control the "tone" of a sound on the piano by what we do after we press the keys just made me want to cry.

This is one of the first things I teach every student. We create the illusion of "tone" by how long we hold notes (often with the pedal) and the balance of sound between the fingers (voicing). That's it. The hammer is projected towards the strings by how quickly we start it, and once it's on it's way we have no more control over the result than throwing a brick out a winder and thinking we can mentally change the path or result of the brick falling.

But what your fingers do after they pressed the keys, or more precisely, between they press the keys, is some how a reflection on how they pressed the keys. No?

I don't understand what you are trying to say. What do you mean by "a reflection on"???

The quality of the piano has a lot to do with the "ugliness" of sound. There are pianos out there that are incapable of producing any nice sounds, and every single note is ugly.

And there are concert grands out there that have such lovely tone, no matter how hard you bang on the keys, the sound is still beautiful.

That is my belief too.
I suspect less than 1% of piano players understand or believe this.

+2
I also don't believe we control the "tone". Having a relaxed hand and body is good. Tension leads to pain and injury. If your student prefers the tone when he has a relaxed hand, maybe it's because he's feeling better when he's relaxed.