Joining two fishing lines into one, in the history of fishing is very important. The Willis knot is therefore used to join the leader line to the lead-core fishing line. While using a mono leader line, just one handmade knot is enough. Whereas while using a braided line leader (Fireline, or Spiderwire), it’s safe to use two handmade knots for a tight grip. The knot holds the two lines together with the handmade knot giving it the last security needed just to be safe. The knot is therefore deemed extremely reliable by fishermen who use it. Alternatives include Double Uni tied with the sheath material or an Albright special.
Here are clear instructions on how to tie The Willis Knot:
Using your finger together with the thumb, squeeze down on the lead core line, and then pull back four to around six inches of the sheath. Cut off the inside wire right there, and then straighten the sheath with nothing in it.
Tie a very loose handmade knot on the empty sheath material. Slide monofilament leader line into the sheath, pushing it back up until it comes into contact with the inside lead.
Now work that handmade knot back near the end of that sheath while slowly tightening the knot. This should happen while the mono is in place, do not move it.
Finally, test the knot by pulling very hard on the leader line. It shouldn’t slip at all. It doesn’t seem as tight, but the connection is so tight and tightens even more when pulled.
Always test a knot before using the whole line to fish. This is important in identifying if a knot is tied properly to avoid the knot untying itself when you are busy pulling out a fish. You can do this by pulling harder on the leader line.