My form really suffered until I moved my thumb out of the way and now use it to solidy my anchor by placing it behind the jaw bone. Previously the thumb would hit my cheek bone and it would become impossible to get to anchor, and I would find myself collapsing at the shot.
The other important aspect of the string hand is to remove tension from it. You want the tension in your back muscles, if you tense up your string hand, you are also tensing up your bicep. Using a hook method you can easily get your thumb out of the way and get a straight string hand and reduce the tension.