What are they called?

It is commonly referred to as a "hard thumb" and is made from a thermal plastic splinting material. The plastic material is used in the medical field to make splints. Contact a medical supplier. The material comes in a sheet, you cut a piece large enough to go around your thumb, heat it in hot water, mold it around your thumb and hold in place while it cools. You then trim your "hard thumb" to fit. You can leave this in your catcher's mitt or remove it. If you use a batting glove under your catcher’s mitt, I suggest you wear that glove when you mold your hard thumb. Can be reheated and re-molded.
Hope this helps,
Fungo

They are called Direct Protect. They also position the bat in the hands properly.

www.prohitter.com

Ok, thanks for the help