In a double bridle arrangement, the curb or Weymouth serves to assist in achieving maximum flexion from the horse while preserving a light contact. The curb acts on several parts of the horse at once to achieve this result. In the mouth, the curb puts pressure over the tongue and bars, which can be more or less severe depending on how tight the curb chain is kept, how much of a port the mouthpiece has, and how long the shanks are (a too tight or too loose curb chain, a higher port, and longer shanks are all more severe).
The curb uses a leverage action, since it attaches to the cheek and headpieces of the bridle at a point above the mouthpiece, and the reins attach to the end of a shank (essentially a lever) a few inches below the mouthpiece. When the rein is engaged, this lever action puts some pressure on the poll of the horse through the cheek and headpiece, and on the mouth and jaw through essentially squeezing the lower jaw between the mouthpiece and the curb chain running under the jaw.
A tighter curb chain limits the amount of angle that the curb shanks can travel, thus causing less pressure to be put on the poll and jaw. However, this does mean that the action comes into play more immediately upon contacting the rein. The ideal point at which the curb chain should engage is when the cheeks are approximately at a 45 degree angle to the mouth of the horse.
In any case, it should be clear that the curb is a very sharp instrument and should be used only in the hands of those that have a very sensitive touch.
Loose Ring Snaffle Bit
The loose ring snaffle is one of the most common types of snaffle cheeks, and generally considered to be the best style of cheek for disciplines requiring sensitive contact through the reins, such as dressage. Because the ring-shaped cheek pieces are attached to the bit by running through holes bored into the ends of the mouthpiece, the mouthpiece to move freely in relation to the rings. This allows the mouthpiece to move more independently with the tongue and jaw movements of the horse, even when the reins are maintaining pressure on the bit. This is generally considered an advantage in disciplines like dressage in that it encourages a relaxed jaw and mobile tongue, but some horses can find this freedom overly stimulating and get too playful with the bit, especially when there is no rein contact helping to stabilize the bit. The rings are also able to swivel freely in a lateral direction, allowing for clear transmission of direct rein aids, which is particularly useful with young horses. Most cheeks used in snaffle bits are able to swivel laterally, but as the name suggests, the loose ring has the least resistance in this respect.
There are two possible downsides to the loose ring snaffle that may be relevant in certain cases. Firstly, the gap between the rings and the holes in the mouthpiece can pinch horses with sensitive loose-skinned lips. This can be a particular problem if the bit is sized too small for the width of the mouth, or if the holes in the mouthpiece are poorly bored such that they have sharp edges or are significantly larger than the rings going through them. Secondly, the rings provide only limited resistance when the bit is pulled laterally through the mouth, and when pulled hard, the bit can go right through the mouth, rings and all. The larger the rings, the less of a problem this is, which is why special training bits are made with extra large rings. With more advanced horses who do not need significant help from the bit when turning, smaller rings are generally fine.
Information from thebitguide.com