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.
A horse shaking its head when bit pressure is applied or trying to put its tongue over the bit are classic signs the horse has a sensitive tongue. With this in mind the McHardy was created, it has a medium port creating space for the tongue, relieving pressure. The Buster Roller in the middle increases bar pressure and as a result extends the head and brings the nose in giving more control.