I think those are the extra attachments that usually come with Flexi leads. I haven't bought a Flexi in a long time since I found out how dangerous they can be, so I could be remembering wrong too!
I never used it since I believe it's something that goes around the dog's neck in case the collar breaks and we always attached to a harness...I could be way off here.
ETA: Ah ha! I found the instructions, and they refer to it as a safety collar!
http://www.flexiusa.com/operation/safety-advice.php
I should add that a coupler typically has a d-ring at the "point" on the V shape which attaches to the leash and each end of the V shape has a clip to attach to the dog. My favorite coupler is by Wacky Walkr.