Half Cheek Bit
Also called the “half spoon” snaffle, this type of bit is a relative of the full cheek snaffle, because it has an arm extending down the side of the cheek from the mouth. Unlike the full cheek, it only has this lower arm and it is shaped in a somewhat flatter, spoon-like shape. The principle is the same, however, and that is to provide increased lateral action when direct reining is used for turning, and to prevent the bit from being pulled through the mouth of the horse. This bit is typically seen with an eggbutt style of connection at the mouthpiece, making it also a good choice for preventing pinching at the corners of the mouth.
With only the lower spoon, the bit is somewhat safer than the full cheek, in that there is less likelihood of it getting caught up in anything, and it does not require a restraining loop on the bridle. Because of this, the bit is a popular choice in driving and somewhat in racing. As with any arms, however, there is still danger of the spoon being caught up in other tack, wire mesh, and the like, so it is important to keep the head of the horse free from possible entanglement.
Information from thebitguide.com