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 Bombers Control Plate is set at 45° to follow the contour of the horses tongue. Consequently, a horse who uses his tongue as a means of pushing against the bit and therefore taking control, will draw back and tuck his chin in, coming back to a better point of control.