Powerful sound

Seeing that other pianists have very fast and crispy chord attack sounds, sometimes we mistake that as using a lot of energy. Actually, the loud sound, the sharp sound, or the crisp decisive sound doesn't come from more energy. It is indeed coming from a different angle of attack as well as the speed at attacking the keyboard. The angle is a bit more vertical, the speed is faster. But, NOT more energy is applied. Also, the arm muscle needs to be relaxed. A tensed arm is like an arm with a lot of burden. Imagine a person can never run fast with a lot of burden. To be fast, you need to mentally imagine/choreograph a smooth path for your arms+hands movement. Relate your sound to your physical sensation (hand movement/touch of keyboard/feel of distance). Also a clever way of intensifying the sound without spending more energy is actually by using your arm weight. You can try dropping your arm weight at a certain angle and speed to achieve the desired dynamic of the sound. It is a fun experiment to do at piano to explore the correlation between speed and loudness of the piano sound. Then you will realize it would be wise to adjust certain habits so that you are using speed but NOT more energy for loud sound. That way you make yourself more "energy-efficient" in performing music.