A leash is attached to one end and as it's jerked, the chain slides through the opposite end, which causes the collar to collapse around the dog's neck like a hangman's noose. 

 A steel choker is preferred by most users because it can withstand repeated strong jerks without breaking; they are also made of nylon, cotton and other materials.  However, all chokers have the same function: they choke the dog.  Why do you think it's called choker?

 They're used on all types of courses, including obedience, behavior, protection, guard, attack etc...It's actually crushing the esophagus while cutting off the flow of blood and oxygen to your pet's brain.  Effective, but extremely painful. 

 When applied, the choke collar is very cruel!