Rolf Gauger has an article in Bowling This Month
, October 2001, where he talks about setting the thumb for a quick and clean release so that the fingers can do their job of creating a rev angle and a greater rev rate. Here's the essence of what he mentioned there: The longer the thumb stays in it's hole, the weaker the ball will be at the pins. Some bowlers suffer from a knuckled thumb where the knuckle rubs at the back of the thumbhole. This slows down the release which causes a reduction of revs and often an overturning of the wrist/hand. This may also create a late release, hanging thumb and sometimes injury. To combat this, bowlers try to keep the thumb straight which may result in an inconsistent release, often with the thumb slipping out prematurely and drop the ball. A much better solution is to set or lock the thumb. This is done by inserting the thumb into it's hole straight, with the thumbnail lightly touching the back of the hole. Then - here's the important part - pressing the skin at the base of the thumb against the rim of the hole and pulling the thumb a tiny bit back out of the hole, just barely enough to make the skin wrinkle up and lock itself against the rim.
The thumb must remain straight during the operation. If the thumb is pulled back too far, you'll feel it sliding out. But when the skin under the base of the thumb stays wrinkled, you'll notice that you can actually hang the ball at your side and even bounce it up and down without losing it, in spite of the thumb remaining straight. Using this setting the thumb technique, you can perform your normal swing and release with the thumb straight throughout the entire Execution
. He claims the release is virtually as effective as a totally thumbless release, with the added advantage of greater accuracy and consistency.