This item is a safety lanyard.

Quite simply, it used primarily by arborist to secure themselves to a tree in a work positioning situation. The lanyard typically attaches to steel mounting rings (typically called "D's) that are on each side of the work saddle worn by the climber. The lanyard attaches to one side of the harness, then passes around the tree, and back to the other side of the saddle. The clip on one side makes it easy to attach and remove from the saddle as the climber moves from point to point on the tree.

The design of this is really pretty simple, there is a heavy duty safety snap on one end and a tight eye splice on the other. To make a low cost but very effective adjustment device, you simply attach a carabiner and pulley to a piece of prusik cord on the lanyard and you are all set.

The photos above show what we are talking about.