When it comes to piano technique, that is, the physical aspects of playing, you will find the discussion to be almost entirely about the fingers. Thousands of exercises have been written and given to piano students to, supposedly, strengthen and/or train the fingers. Most of the traditional methods barely address the use of the arm. In recent years, thankfully, there have been more teachers who understand the use of the arm in technique.
However, hardly anyone talks about the torso.
In a sense, the torso is the foundation of technique. The shoulder, which is a ball-and-socket joint, enables our arms to cover the entire width of the keyboard and to move in every direction. It enables us to play octaves and myriad other types of technique which the hands and fingers could not possibly do by themselves. The shoulder joint is the connection point between the torso and the arm.
You might assume, therefore, that the torso just stays inert while the arms do the work. But this is not the case. For example, suppose you are playing a passage where both hands are in the lower register of the piano. If you were to keep your torso inert, right in the center, you would have trouble playing in that register without a significant twist in your wrist in your right hand, the one that has to reach farther. Likewise, if you are playing both hands in the upper register, the twist would be in your left hand. In order to play these types of passages successfully, and more importantly, without stress or strain, it is necessary to shift the torso towards the left or right in order to get the torso and arms in better alignment. If you shift the torso, you will see that twist in your wrist disappears. If your torso and arms are better aligned, you will have more power and better accuracy. I call this your "stance." You might think of it as where you are "centered" relative to your arms. Your torso must anticipate where your arms need to be next. Most of the time, of course, your stance is the center of the keyboard, but definitely not all the time.
You almost certainly need to adjust your stance, at least slightly, when you are playing hands crossed. Otherwise you will again have the twist in your wrist.
I recently have been playing the Bach-Busoni D minor Chaconne. In one section the two hands are in octaves in the lower register, then rapidly jump to the upper register for some big chords, all very loud, and back again. It would be impossible to do without shifting the torso back and forth. Of course I was aware of this yet still kept missing a few of the chords. I discovered that I needed to shift my torso just an inch or two more to the right than I had been doing for the chords in the upper register. Immediately the problem was solved! I had been close to shifting enough, but the additional tiny amount made the difference between missing notes and not. Amazing! It didn't take any extra hours of practice, but instead, an awareness of what the torso was doing.
If you have to shift your stance over to either side by a significant amount, you may feel you are off balance, almost like you could tip over. This is easily solved by moving your left leg farther to the left to balance yourself. You wouldn't be able to move your right leg, since it would be pedalling. I've noticed people rarely talk about how your legs can help you. Once your become more aware of them, you'll find slight adjustments have a positive effect on feeling really "grounded" at the piano.
You virtually never see a great pianist whose torso is inert. The torso wants to move with the music. As an experiment, play with a completely rigid torso and you will find it more difficult to play expressively.
It is important not to slouch or hunch over at the piano. While you'll see pianists who do hunch over, it can create back and neck problems over the years. How you sit will affect all aspects of your technique. See my earlier post entitled "First Do No Harm" for more about this important subject.
