|A Cod End Knot|
knot has a very specific function. It's one way of tying up the
bottom end of a trawl net. The gray squares represent where the
rope is emerging from the net into which it is threaded.
It's especially useful in this function since you can see that it doesn't require access to the ends of the rope. You can imagine how much extra rope you might have once you tighten up the bottom of a trawl net.
Push a bight of rope through a half-hitch as shown in the first diagram and pull another bight (arrow 1) through the first bight and tighten down the first bight to trap the second bight (arrow 2). In the second diagram, you pull another bight of rope through the previous one and trap it by pulling on the other line as seen by arrow 4.
You can continue this "braiding" method to take up the extra rope if you like and tuck the ends through the final bight to secure it. When you want to undo it, you'd untuck the ends and pull alternately on each line to unzip the knot.
I've heard that it's also common to expend the extra rope by coiling it up around a bight in a manner similar to that seen in the Hangman's Noose.
Related pages: Electrician's Braid, A Duffel Bag or Pouch Knot