Piano scales finger.
In order to obtain this smooth passage of the thumb in scales, I advise that the wrist always be kept absolutely loose, and that in slow practice, when the thumb is ready to pass, the wrist be raised temporarily from its usually low position to a higher one; also the finger which strikes the last note before the thumb has to pass (in scales it is always the 3rd or 4th finger), should be slightly inclined towards the direction in which the hand is going to travel. Taking the ascending scale of C major, in the right hand, for example, and illustrating what I want to point out by a diagram thus :
Ascending right hand.
How to pass the thumb : fingers | hands in scales
It will be seen that upon the E, which is struck by the 3rd finger, the line underneath is raised and inclined towards the direction the hand has to go, so as to represent the lifting up of the wrist, and the inclining of the finger. The thumb then passes easily underneath the fingers on to the next note F, without any awkwardness. The same movement is repeated further up the scale after the 4th finger, and so on through all the octaves in ascending scales for the right hand. For descending scales, the process is reversed. The wrist is raised when the thumb falls, and the finger which follows it is inclined downwards in the direction the hand has to go.
Descending right hand.
Thumb passage | wrist in piano scales
In the left hand exactly the same process is used as in the right, only the order is reversed, that is to say, the wrist is raised at the thumb, in the ascending scale, and at the 3rd or 4th finger, in the descending one, the inclining position of the fingers being correspondingly observed. In all scales in every tonality, this action of the wrist and fingers should be similar, and the principle of lifting the wrist at the finger before the thumb passes, and inclining the finger in the direction the hand is to travel, greatly facilitates this passage of the thumb, and ensures smoothness and freedom of motion. In fast scales this movement practically disappears, as exaggerated actions only impede swiftness and look ungainly, but a smooth and undulating motion remains, which is elegant and imparts an elastic and supple articulation, and also gives character to the various passages.