"It helps to use a shoulder rest for many shifts, especially those from higher positions to lower positions. It can help when playing very high and when the thumb leaves the neck. It can also help our vibrato when playing lyrical passages."
Shoulder rests are a useful tool which help us develop our shifting and vibrato actions. Like all tools however we need to be clear about the best way to use a shoulder rest.
A shoulder rest is useful when playing large descending shifts from high positions such as 5th position down to 1st position. The arm needs to swing back to the left and we do not want the violin to droop down during the shift.
You can do this without a shoulder rest by using compensatory movements of the left thumb. However by balancing the violin with shoulder rest and chinrest the arm and hand can move as one unit during the shift.
A shoulder rest is also helpful when we play very high up the fingerboard. The left thumb needs to leave the neck and go either on the side of the violin (as shown here) or on the side of the fingerboard.
Players with large hands can keep the thumb at the neck almost all the time (although leaving the thumb at the neck for passages such as the high E trill in the 1st movement of the Hindemith Solo Sonata Opus 31, No.2 is a challenge for even the largest of hands). Again in these situations a shoulder rest is helpful since without one we need to briefly raise the shoulder to support the violin.
We also use the support of the shoulder rest when using vibrato (especially arm vibrato) in lyrical passages.
Be careful not to use the shoulder rest as a 'bandaid' solution, that is to cover up underlying issues in body use, finger action or shifting. Make sure that the left shoulder does not push up against the rest and become immovable....