Forearm Rotation for Keyboard Passages

As to rotatory movements themselves (as indicated earlier) these are optional. During slow passages they are not only harmless, but may even be particularly helpful. Whereas, in quick passages they still exist, but are not observable by the human eye, and finger-movements must here be substituted. In actuality, the finger, hand and forearm benefit from the invisible rotational help given individually for each note. Think of it like this, the rotation is visible during slow practice, and these motions become invisible during performance. Whether, in the end, you choose to exhibit rotative movements in a passage, or instead choose finger or hand movements, depends upon which of these three components you place into motion during performance.

Finally, since correct rotation is important in determining whether a passage is secure, play the passage through once or twice so slowly that you can analyze and re-analyze the direction of rotatory-impulses.
Do this so slowly that you can actually see the alternations from pronation to supination. A rotary motion towards the thumb is designated as pronation. Thus you impress upon your mind the direction of the rotational help which eventually you have to provide invisibly and without actual rotatory movement, and yet correct in the incidence of its rotational stresses for every note.

Pronation of the hand in keyboard playing refers to the inward rotation of the forearm and hand towards the center of the body, which allows the fingers to strike keys on the keyboard with the fingertips. In piano and keyboard playing, proper hand position and technique are important for achieving accurate and efficient playing, and pronation is one of the fundamental hand movements used to play the keys. When the hand is properly pronated, the fingers are able to approach the keys at the correct angle and the fingertips can strike the keys with precision. To achieve proper pronation, the player's forearm should be in a relaxed, neutral position with the palm facing slightly downward. The fingers should be curved and the fingertips should be used to strike the keys.
Over-pronation or under-pronation of the hand can lead to tension, discomfort, and incorrect playing technique, so it's important to maintain proper hand position and use the correct amount of pronation while playing the keyboard. Professional piano teachers can help students to develop proper hand position and technique through guided practice and feedback.