The release point at which a bowling ball is thrown plays a big part in the success of the throw. A bowling ball that is released too soon will likely roll straight -- slowly -- and often end up in the gutter. A bowling ball that is released too late will end up overshooting its target and also likely end up in the gutter. The placement of the thumb has much to with how well a bowling ball throw succeeds in striking the pins. Whether you throw a straight ball or a hook ball, releasing the ball properly with your thumb will lead to higher scores in the long run.
Choose the proper bowling ball for your style of bowling. Select a ball that is the proper weight. For example, if you throw a straight ball, you can go with a slightly heavier ball than someone who throws a hook. Choose a ball that has comfortable finger and thumb hole placement as well. This will help ensure proper thumb release when you throw the ball.
Work on your footwork and body motion. Practice taking the same steps and making the same motions with your body and arms. This will help maintain the proper release point for your thumb.
Release your thumb on the down stroke as you approach the foul line. For straight bowlers, the thumb will release and should come up at a slight angle while your index finger, middle finger and ring finger form a "W" that points directly at the pins. For hook bowlers, your thumb will release and rotate your fingers two inches like you are making a handshake motion, with your thumb pointing straight up to finish the throw.
Tips
Remember that your thumb release from the bowling ball should happen while your fingers are still inside their finger holes. The fingers push the ball down the lane, not the thumb.