The Rapala knot is a very strong knot used for attaching the leader to a fly or lure. The advantage of using a Rapala knot or other loop style knot is the fly or lure is able to swing on the loop adding more movement to the fly when the fly is swinging through the pool. It is an easy knot to modify the length of the loop when tying it. Go for a longer loop if you are looking to place a hook further back in a tube fly or a small loop for tying straight onto a hook.