The Full Cheek Snaffle has long, extended arms above and below the mouthpiece on either side of the lips of the horse, with a ring attached to it. These cheek pieces have a lateral guiding effect (i.e. apply pressure on the sides of the face), and also prevent the bit from sliding through the mouth. The full cheek is often used with bit keepers to prevent the cheeks from getting caught on anything, keepers also, perhaps more imporantly, stabilise the bit and keep it in the right position for the correct bit action within the mouth. Bit keepers make the bit more fixed in the mouth than a loose ring snaffle.
Bit keepers can be attached around the entire cheekpiece or only partially, as shown. When attached as shown they are more likely to break free in the even the bit did become caught on something.