Not sure if that is the correct term for the piece of leather or rubber that you use to hold the full cheek tilted back. Since I am bad at describing stuff here is a visual:
I have always been told that you HAVE TO HAVE that piece to hold the bit at the correct angle. However, I have heard more and more people say that you don't really need to have that piece.
So what exactly does that piece do, and do I need it?