Eggbutt Lock Up from Bombers.

The eggbutt prevents the horse from being pinched in 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 a little 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 on the tongue and bars when you shorten the reins.