Is the left hand thumb a lever?

Question: "The thumb is not squeezing but does it act like a lever? If so, the thumb itself is somewhat relaxed but there is a pressure/support on the neck that comes from the back/upper arm. Is that correct ?"
Answer: That's right, generally the pressure to contain the strings comes from the expansion of the back, transmitted via the arms. Nothing is absolute, however, of course the Left Hand will need to grip from time to time. What we are exploring here is "how much can we minimise the gripping of the left hand?" And the answer is for a lot of people - "quite a lot!".

Also, using the bigger back and arm muscles to produce pressure on the strings allows the fingers a lot more freedom of movement than is the case when "gripping".