The Eggbutt is a very common multi-discipline style of cheek piece for snaffle bits. The eggbutt snaffle minimizes two problems that can arise with its cousin, the loose ring snaffle, whose rings can pinch the edges of the horse's mouth, and which doesn't provide much lateral stabilization. By flaring out the ends of the mouthpiece and joining the rings with flush swivel joints above and below where the lips contact the edge of the bit, the eggbutt can be a more comfortable alternative for many horses. The edges of the mouthpiece are less likely to pinch the horse's lips, and because the cheek is fixed in relation to the mouthpiece, the bit offers moderate lateral control.
When the bit is pulled laterally through the mouth, there is some resistance on the opposite side, which can help encourage the horse to turn with less danger of pulling the bit through the mouth than exists with a loose ring snaffle, though more than with a dee-ring or full cheek snaffle.
By having rings fixed to the mouthpiece, the eggbutt does give up some mobility, in that the position of the mouthpiece is more influenced by the movement and position of the cheek pieces than by the movement of the horse's mouth, unlike the case with a loose ring snaffle.