The direction of the ball is determined primarily by how and where the arm swing brought it forward. But, the ball usually also contains some spin or "side-rotation." Putting a side-spin on the ball is done intentionally by better bowlers. Beginners may do it accidentally and unpredictably. In either case, the way the ball is rotating usually affects its movement. If the ball is rotating in a counter-clockwise manner it will move right-to-left. 

Left-handers put a clockwise rotation on the ball to have it hook from left-to-right. In the past bowling balls were made of hard rubber or plastic. Nowadays urethane-shelled balls have become popular. In general, urethane balls react more to the spin on the ball. That means they hook more and deflect less when they hit the pins.

A hook that is predictable is a big advantage but for newcomers, most progress is made by improving timing, balance and overall accuracy. Even so, it is not too soon to learn the hand position for a hook release.

Specifics of the Hook Release

To make a ball hook, the thumb exits first, followed by the fingers. As the ball pulls itself off the fingers, it starts rotating. The direction of the rotation is determined by where the thumb was pointed as the ball left the hand. This is a split second process. Having the thumb come out first is natural because the thumb is shorter in length than the fingers. Nevertheless having a well fitted ball is certainly important. So is making a good approach to the foul line.