"If we only played the broomstick, we would only need to use a piston shifting action. However, since we play the violin, we can only use the piston shifting action in the lower positions and in the higher positions the arm needs to move around the violin."
To shift to higher positions swing the whole arm across the body to the right, and release the contact of the index finger on the violin neck. Anchor the thumb at the throat of the neck to help in shifting down. To shift down, swing the arm to the left and replace the contact of index finger on violin neck.
When we shift beyond 7th position the hand extending from the wrist becomes the dominant shifting action and the arm position changes only a little.
When shifting up very high, we may need to release the thumb anchor at the throat of the neck. In this case we can let the thumb rest on the side of the violin or on the right-hand edge of the fingerboard. Players with smaller hands need to do this more often than those with larger hands. When descending the thumb leads, taking the hand back to the violin neck.
Shifting - Thumb as Pivot; Stretches & Contractions