As it is a slip knot it is one of the few fishing knots jigs can move freely on if the noose is not overly tightened.
This knot can be tied as a double uni knot or a uni to uni knot the same as a conventional uniknot. Diagrams and video on how to tie a slip knot are below.
Allow yourself plenty of line in the first loop and start as you would for a four turn clinch knot. (see slip knot pictures below)
I have made the long tail (tag end) red so you can follow the next step. Put the tag end back through the front of the loop and do another four turns up the inside of the loop following the lay of the original four twists.
To tighten as a noose put
your fingers in the loop the swivel is shown on, and pull the knot up slowly.
If any of the loops jump ot of place give the tag end a light pull.
Rotating the hand you are using to hold the loop open while setting the knot will make it set much more eaisily.
The hangmans knot can then be cats pawed to a clip or swivel and the noose tightened
See the finished catspaw uni knot here.
Paul's Fishing Kites uses this knot extensively for those critical joins in kite fishing rigs that have to be both strong and hardwearing. It is also one of the better surf fishing knots.
Up to seven turns can be used but make sure to put the same number of turns in both sets of twists or the knot will not pull up correctly.