The long cheeks press against the sides of the muzzle during lateral movements and prevent the bit from being pulled through the mouth. The cheeks can be left free, or they can be held in position through the use of FM Keepers. 
If keepers are used, the bit acts not only on the corners of the mouth, but also on the sides of the bars, exerting more pressure. The loose rings allow movement of the mouthpiece (as opposed to the Full cheek Eggbutt bit).