The Eggbutt cheek piece prevents pinching of the lips and gives a solid feel against the side of the face. It also prevents the bit being pulled through the mouth. The release is slower than the loose ring and also introduces light poll pressure.
The Happy Tongue Straight is a solid mouth bit which is ported to relieve tongue pressure but is not curved. This makes it slightly harsher than the curved Happy Tongue. The swivel mouthpiece can rotate on the horizontal, allowing the cheek piece to move independently, thereby the horse can set where it finds the mouthpiece most comfortable… removing tongue pressure.