Thumb under a Finger in a passage

The rule applies with equal force when you turn a finger over the thumb, unexpected although it may seem. Therefore, to help a finger turned over the thumb (for example, when the left hand plays an ascending passage), you must still give the invisible or visible rotatory help outwards after finger 3 crosses over the thumb.
Thus, when playing a trill played with a finger turned over the thumb, you should give alternate rotations

1. inwards (towards the thumb) and
2. outwards (towards the 5th finger).

The motions are continuous, visible and smooth during practice and invivible during performance.

Much scale-playing is uneven because this rule has not been grasped. This rule, however, is precisely reversed when you turn a long finger over a short one. Hence, when the middle-finger is turned over the fifth, you must rotate with the middle-finger towards the thumb side.