Hackle Pliers are used when winding hackles onto a fly. If you try to grip a feather between you thumb and index finger and wind the hackle on the fly, 9 times out of 10 the hackle will slip after you release tension on the feather without knowing. Hackle Pliers get around this probem, they grip the feather and allow you to perform a hackle winding without any loss in tension. There are namy types of hackle pliers , some are even rotary ones. The two that I tend to use are a spring loaded one (Yellow) and one where the body of the hackle plier twists to anchor the feather in place and twists in the opposite direction to release it.