This knot is used to tie a leader butt directly to your fly line. Originally, a nail was used to tie this knot–with the leader being inserted in the small space between nail and line.

By using a small tube instead of a nail, it is much easier to insert the leader back underneath all the wraps. Any small-diameter tube can be used–a hypodermic needle, the refill barrel of a ballpoint pen (cleaned out, of course), etc.