You will have to put it in all balls that you use.  I tried it once, and thought I would just try my one ball, but when I grabbed the spare ball, I couldn't throw it.  Yes, to me it made that big of a difference.  Be Careful

I used them for a while, and then on my next ball drilling decided to try throwing without them. Ended up tapping the hole open more to fit the carpet in. Great product.
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There is nothing so easy to learn as experience and nothing so hard to apply.

Thank you all for your replies....

Where do you guys put it? in the back? I don't know the only balls I've seen had it in the back of the thumb, I never cared for it. I just like my slick thumb slugs.
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Throwing hambones since 2005.
www.myspace.com/nestes12

I use Magic Carpet in all my balls. You need to use it in all your balls so they all fit and feel exactly the same. Because of the way the carpet works, it gives you some tolerance when your thumb swells or shrinks.

You have to open up the thumb hole on the back side before you put the Magic Carpet in, because if you don't it will make your thumb hole too tight.

It is a wonderful product, allows a clean release no matter how your thumb reacts to the heat, cold or pressure. No pop, as the fibers allow the air to circulate.

Love it!!!!
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Never take a sleeping pill and a laxative on the same night.