I see this constantly and wish I didn’t see it at all, ever. It causes so many problems – poor roll, squeezing the ball, killing the shot, dropping the ball, no backswing, grunting at the foul line, etc. Unless you are making a rather sophisticated adjustment, you should always put your thumb completely in the ball. Sometimes people are afraid of sticking in the ball and don’t put their thumb all the way in. Your hand was measured by your pro shop professional and your span gauged with your thumb all the way in. If you don’t put it all the way in, you won’t be able to free swing the ball, will have to control it, and the pins always know when your armswing is tight. I understand that a very common reason for not putting the thumb all the way in the ball is that you can’t because you are so stretched. That’s another article.
Put your fingers into the ball first and then your thumb. Putting your thumb in first will usually have you feeling like the ball was drilled for someone else! Super tip – This sometimes works well for straightening the ball out for your spare shots.