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 Elliptical Lock Up removes the nut cracker action of the conventional three piece, meaning less pressure on the bars but more on the tongue. The Elliptical Lock Up is therefore a softer option, than a conventional elliptical, although stronger than a control plate.