Page 94 - Joseph B. Healy "The Pocket Guide to Fishing Knots"

Needle Nail Knot

This is a similar premise to the Nail Knot, only now you use a needle to
“sow” your leader into the fly line. This is a great way to attach a leader
butt section to the fly line.

1. Start by threading the line into the center of the fly line.
2. Wrap the line around the fly line, using the needle as a base, as you
would a Nail Knot. Pull the tag end under the knot wraps, using the
needle as your aid.