The Pelham bit is often used for horses that are too strong for a snaffle.
With the Pelham bit, if the large ring is used by itself with one rein, it acts like the lifting action of the snaffle, upward against the corners of the lips. If the bottom ring is used independently, we have a curb action. If both rings are used with two reins, the action will be somewhere between the two.
Basically the Pelham is an incorporation of the Weymouth and Bradoon, and should theoretically be used with 2 reins, the first rein is attached to the snaffle rein, and the second rein attached to the ring at the bottom of the shank. When the first rein is used the Pelham acts like a hanging cheek snaffle, putting slight pressure on the poll and various pressures in the mouth depending on the mouthpiece, when the second rein is used, this increases the leverage and lip pressure, and also the curb chain comes into play. The greater the distance between the cheek slot and the mouthpiece the greater the poll pressure, the greater the length of shank the greater the amount of leverage. The curb chain should be fitted so that the curb chain tightens when the second rein is brought into play, but there must be a bit of give so not to tighten too much..