This type of collar is an interesting concept where it tightens when one end is pulled either by the dog or by the owner and loosens when both owners and dog are relaxed. They come in two main materials; one is a chain-linked choker used by a lot of trainer who believe in old-school techniques of force-based training. The other type, also known as slip collar is made of leather, nylon or other softer material.
Pro: Slip collars are used by newer handlers and trainers and highly preferred in the show ring. Choke chain is used for compulsive pullers.
Con: Can be fatal if not used properly or with negligence.