The Eggbutt cheekpiece prevents pinching of the lips and give a slightly more solid feel against the side of the face. It also prevents the bit being pulled through the mouth. The release is slower and also introduces some poll pressure.
The Lock Up snaffle removes the nut cracker action of the conventional snaffle, meaning it will bear less pressure on the horse's bars. The Lock Up is therefore a softer option, than a conventional snaffle, although it still applies tongue and bar pressure when the reins are taken up.