The Eggbutt cheek piece prevents pinching of the lips and give a slightly more solid feel against the side of the face. It also prevents the bit being pulled through the mouth. The release is slower and also introduces some poll pressure.
The Buster Roller link is shortened which increases bar and tongue pressure making this mouthpiece sharper than conventional link bits. The increased pressure on the bar will lift the head and the roller will increase pressure on the tongue bringing the chin in. Best suited for a horse with a low head carriage and which leans on the bit.
