The Kimberwick (Kimblewick in the UK) bit is a form of Pelham.
The main difference between the Kimberwick and the rest of the Pelham family of bits is that it only uses one rein.
As with the Pelham, the action of the Kimberwick is dependent upon the position of the riders hands. The rein slides on the d-shaped bit ring. If the hands are lowered prior to pressure being applied to the reins, the rein will slide about an inch and the bit will assume about a 45 degree angle in the mouth. This causes the mouthpiece to act in a downward and backward motion on the lower jaw.
This is complemented by the downward pressure on the poll, giving the bit a lowering action.
In the Uxeter Kimberwick, the presence of two slots in the bit ring allows the rider to choose the amount of lowering pressure the bit exerts.
Many horses go well in the Kimberwick, especially going across country, where the bit gives the rider a little more control than in a snaffle.