When you hear your favorite singer, you know who it is instantly by the voice. Even listening to wind players and string players, you often can identify the player by the tone. What about piano? Can you tell who is playing just from the sound they get? The answer is, yes, sometimes.
The secret to getting a great tone out of a piano (or a more pleasing tone out of a lesser instrument) is to support the tone like a singer or wind player supports the phrase with the breath using the diaphragm. String players utilize the bow for a smooth line and spend years developing a technique to get a beautiful sound. So what is the analog to the breath on the piano?
A phrase generally has a rise and fall, just like a breath. If you were to calculate each note of a phrase played on the piano to make each note successively louder to the peak then each note softer than the next to the end of the phrase, you would end up with a very calculated type of playing! The secret is to use a constant weight of the arm which grows to the top of the phrase and diminishes to the end of the phrase. The fingers support the weight and transfers the weight smoothly from note to note. So you exert downward pressure on the key not just on the attack, but throughout the length of the note. This enables you to get a smooth line by using the weight of the arm as the constant like the breath of a singer.
When playing large chords loudly, you can get a beautiful sound by playing from the surface of the keys. If you strike from above with the arms, this creates a slapping sound which is harsh and ugly. By staying in contact with the key and releasing energy to the bottom of the key bed with strength, you will get a beautiful sound no matter how much energy you expend. Try this in these 2 different methods and listen to the difference in the sound. You will be amazed.