This is an assembly made (usually) of wire rope and a rated locking clip used to keep a lighting instrument from falling to the ground should there be a failure in the pipe clamp used to attach the instrument a pipe. It is wrapped around the yoke of the instrument (or affixed to an integral attachment point) and the mounting position (pipe/truss, etc.), allowing slack so the electrician can focus the instrument. They are a MUST HAVE for all lighting instruments. Photo from http://www.peaktrading.com/productpages/default.aspx?ProductPageId=1658.