The finger releases to the top of the string, as if playing a harmonic, and the thumb responds by also releasing pressure on the violin neck.
This light touch gives ease in shifting as we have to 'release the brake', while maintaining enough contact with the violin to give guidance in the shift.
Notice that the thumb is in contact with the neck at all times and that even when playing with harmonic touch the finger always has contact with the string. This is important in maintaining a balanced hand position.
Make sure to keep the bow moving steadily as you release the pressure of the finger and during the shift as hearing the slide helps us know when to put the finger down again. Also a fluid action in the bow helps maintain a fluid action in the left arm.
You could practise this release of pressure on any shifting exercise. Here Fintan plays notes from a chromatic scale. Practising shifting from the same starting note to the notes of a chromatic scale is good aural training.
SEE
Shifting - Introduction
Scores - Release Finger Pressure - Chromatic Scale - violin & viola
...