I just started learning cello for about 40 hours, I am really puzzled about whether my left thumb should press hard on the neck, in the same way pliers clamps something, meaning that the thumb apply the same amount of force as the fore finger pressing the string.
I think it has some effects:
If the thumb apply no force at all, with the other finger pressing the string, the cello body would lean back a little, so between pressing and de-pressing the cello body would bounce, making shifting hard, especial a fifth shift.
If the thumb clamps, vibrato would be hard to do.
And another related problem is the horizontal position of the thumb, do the thumb stay at the same position when playing different strings, or should it be a little left when playing on A string, and a little right on C string?