The kimblewick bit is a type of curb bit that has a very mild action.
It has D shaped rings and some varieties have plain rings, whereas the uxeter version has slotted sides. Hooks are usually attached near the top of the bit to attach a curb chain. Mouthpieces can vary between metal, plastic and rubber with all the same variations that you would expect from the snaffle range.
Kimblewicks act on the bars of the mouth, put mild pressure on the poll and also have curb action if used with a curb chain.