Do you mean like moving the whole tronco to the right or to the left ?
That would be one way, but no, I meant the torso is supposed to be pivoting around the "sitting bones" slightly to the left or right (like you would do if you were swaying to the music at the Munich Oktoberfest). Don't move the whole torso by sliding on the bench.
Sorry, it's my english fault but I don't follow you
What do you mean by "at an angle" do you like my whole right arm is in this position: \ this my albow is a bit raised and far away from the center of the body?
Yes. Say, you are sitting in the middle and you have to play in the far treble. If you do not pivot your torso, you will have to extend your arm to reach this area. The arm will then be at an angle with the instrument (about 45 degrees). Keep the hand aligned with the arm and play this way, rather than rotating your hand so that the fingers are parallel to the keys.
Anyway I tried to apply a little finger orientation to all that old passages I played with a thumb orientation but I wasn't able to
For example there's this passage right hand on a 4/4 bar where all notes are demisemiquavers at the speed of crotchet = 100 and it goes like E (above the middle C) - G - C (above the middle C) - D - C - G - E - G
Anytime I try to play this passage my hand is aligned with the thumb in a perfectly straight line, aligning it with my litte finger is almost impossible, either the elbow is in front of my abs or the thumb is out of the keyboard
In order to resolve this, you have to move the hand in and out of the keyboard. For example, when playing the simple pentatonic scale CDEFG, the hand has to move in and out of the keyboard in order to place the individual fingertips roughly in the middle of the white portion of the keys while not destroying the natural curve of the fingers and while avoiding radial or ulnar deviations. You see, one way to get the thumb to play the C is to do a radial deviation, but you can also simply move than hand in, so no deviation will be necessary. Then, when you are going to play D with finger 2, you move out again, and so on. All this works at any speed, but requires a lot of practice. Always imagine where you want to hit the key. Then, while maintaining the alignment of the hand with the forearm and maintaining proper curvature of the fingers, constantly move the whole forearm so that the fingertips end up where they have to be.
This is very well explained and demonstrated in the Taubman videos. They are very expensive, so get your University to buy them if they don't have them already. I think it's required material. Before your Music Library is going to acquire Lang Lang's latest records, make them buy something useful
