Rotation Purpose

In the meantime, do not forget to bear in mind the ultimate purpose of these rotational stresses and relaxations. Whether allowed to become visible as actual rotation along the axis of the forearm, the purpose is to help the fingers in their application of energy towards the keyboard. Work by correct rotational stresses, and not to impede them by wrong ones. Therefore, do not forget to exert the fingers themselves.

Forearm rotation is a technique used in piano playing to help players achieve a smoother and more controlled sound. The purpose of forearm rotation is to allow the player to move their hand from side to side on the keyboard while maintaining a relaxed and natural position.
When a pianist uses forearm rotation, they rotate their forearm to the left or right while keeping their wrist and fingers relaxed. This allows the hand to move along the keyboard without the need for excessive finger movement or tension, which can lead to fatigue and strain.
By using forearm rotation, a pianist can also achieve a more even sound across the keyboard. This is because the rotation helps to distribute the weight of the arm and hand evenly across the keys, rather than concentrating it on one or two fingers.
Forearm rotation is especially useful for playing fast and complex passages, as it allows the player to move quickly and smoothly across the keyboard without sacrificing control or accuracy. It is also helpful for playing chords and other complex harmonies, as it allows the player to maintain a consistent hand position while playing multiple notes at once. Overall, the purpose of forearm rotation in piano playing is to promote a relaxed and natural hand position, improve control and accuracy, and allow for smooth and even sound production.

Remember, the rotatory motions of the forearm must always be applied without an antagonistic muscular pull. That is, always without the exertion of both "pronation" and "supination" muscles at the same moment. In plain English, apply rotation without exerting the forearm rotationally both outwards and inwards at the same moment, which would create a technique-destroying muscular conflict. You must focus to send your nerve-message solely to one set of musdes, and must not allow some of it to leak or stray over to the opposite or "antagonistic" side.

Such antagonistic action is likely to arise in passages where the two hands move by similar motion melodically.
Here the rotation must be implemented for the most part in contrary direction in the two hands. Hence, such passages are often found to be easier to write, than to play. The direction of rotation is confused by the two hands moving alike melodically and our ear actually may mislead us here, as a result "stiffening" may ensue. This difficulty vanishes forthwith when this rotatory contrariness is duly recognized.
Moreover, this tendency to make "difficulties" may also be replaced when the two hands are not played simultaneously, but when a melodically-alike pascession sage is repeated in close succession by the two hands, as in a canon.