Magic Carpet fills the gap in your thumbhole to make it fit properly. I used it for a while until i knew the feel I wanted and could get a ball drilled like it. It is great if you have a driller who overcompensates when taking out of a thumbslug or hole.

It snugs up the thumbhole so that you wont squeeze as much to hang onto the ball.  

A lot of bowlers have their thumbholes drilled up larger so that when the weather conditions change or the body goes thru some changes and the thumb swells, they can still get out of the thumbhole.  In the meantime they have to put in various amounts of tape to keep the thumbhole snug enough not to lose the ball in the backswing or to prevent bending the thumb or squeezing the ball.  

Magic Carpet consists of fibers like part of velcro to fill in some of that space and still allow air to move around the thumb to facilitate a clean release without the suction that would occur in a tight thumbhole.

You can find Ron's email address on this webpage so you can email him and ask politely for a sample:  http://www.bowl4fun.com/magiccarpet/magiccarpet.htm