"If the thumb is pressing release the tension and place it underneath the neck at the nail joint. This will help you monitor the tension in the thumb."
Release tension in the thumb by placing it under the violin neck and playing a few notes. You will easily feel if there is too much pressure from the thumb!
The thumb acts as a counter balance to the fingers both for fingerfall and when shifting. Excessive tension or gripping with the thumb will compromise your facility, coordination, articulation and sound!
The outside of the thumb is quite bony and any excess pressure from the thumb against the violin neck is keenly felt by the player, quickly showing up excess thumb pressure.
Practicing with the thumb under the neck is also useful when developing the hand vibrato in lower positions.
Shifting - Introduction