There is a lot more to a thumb slug than what Erin is saying. The main reason to use them is that when a thumb slug is not used with a reactive ball, a horrible reaction takes place usually some place in your second or third game. The reaction is caused by heat of your body and the material used to make the ball. When this occurs, the ball will not come off of the thumb and it gets worse with every shot. It is because of this reaction that ball manufacturers recommend the installation of a thumb slug with every reactive ball. Have you ever seen someone try and throw a ball and the ball goes up instead of down the lane or somewhere that it should not? A properly fitting thumb slug will give you a great release every time. Thumb slugs are made of urethane plastic, not reactive material!
This is true, but depending on the fit of the thumb some bowlers dont experience it at all. This happens to me frequently, but I have been using baby powder to counter it for years. It acts as a lubricant of sorts for my thumb.