The hackle pliers are used to make it easier to wrap the feathers around the stem of the hook without the risk of breaking them.

Using them is very simple: simply tie the feather to the hook with the tying thread, lock the other end with the hackle pliers and gently wrap the fly tying material while holding the handle of the tool. When you're done, just let the hackles pliers hang still attached to the feather, fix it all calmly with the tying thread and cut the surplus.