When playing without a shoulder rest the left thumb needs to be more active than when we play with a rest. These compensatory movements often involve either the thumb anticipating the shift and moving first or the hand and finger shifting first leaving the thumb at the neck as a pivot for the hand until the shift down from 6th position to 3rd position is completed.
Here you can see the use of the thumb when shifting without a shoulder rest between 3rd and 5th positions.
Rest the thumb at the throat of the violin neck to give security when you move the hand around the body of the violin and back.
On the way up to 5th position the thumb moves in advance of the hand and finger. This is to balance the violin while the hand and finger move up to 5th position.
On the way down to 3rd position use your thumb as a pivot. Keep the thumb in place until the hand and finger have settled in the new position. The thumb then moves down to 3rd position as well.
Fintan normally plays with a shoulder rest but he believes practising without one is good to help understand how the thumb can be more agile in general, even when playing with a rest.
Resources - Bibliography
- for Principles of Violin Playing and Teaching by Ivan Galamanian, p.24