Eggbutt Lock Up from Bombers.

The eggbutt prevents the horse from being pinched at the lips/corners of the mouth by the bit rings and provides a more stable feel in the horse's mouth. It also prevents the bit from being pulled through the mouth. The release is somewhat slower and it also applies slight pressure on the poll. The Lock Up mouthpiece prevents the nutcracker effect of regular jointed bits, which reduces pressure on the bars. Lock Up is therefore milder than a regular jointed bit, but it still applies pressure to the tongue and bars when the reins are shortened.