One of the first controversial pieces of equipment in this section, this german collar is very popular with Schutzhund trainers. Without getting heavily in to the pros and cons which you will find on other sites, I personally have found a prong collar to be an invaluable tool when walking dogs, a prong will stop any lunging and pulling behavior. I call it my "power steering"! It does not need to be used roughly to effect a response, and is a useful correction tool when the dog is high in drive; in protection for instance. In order to work effectively, it must be fitted correctly, enough links should be removed that it sits snuggly behind the ears.
As far as when and how to use a prong and the best size for your dog, ideally an experienced trainer should be consulted. They can show you far more easily than words can explain and they can also tailor the use to the temperament of your dog.
To fit a prong, squeeze a link near the prong end and insert the two prongs into the adjacent prongs loops. This can require good finger and thumb strength with the larger collars. Some collars come with a quick release clip next to the swivel ring. Care must be taken to ensure this is fully shut so the collar does not come off in work.
The prong should be fitted high on the neck. It should fit snug enough to stay in this position. Some links may need to be added or removed for this. This 70lb GSD wears 8 medium links.
The prong collar can be used by attaching the leash to both large rings on the chain. One of these is usually a swivel ring. Using the prong this way, means that it does not tighten and exerts most pressure to the front of the neck.
The leash can also be attached to just the swivel or live ring. This allows the prong to tighten when the leash is pulled and exerts most pressure on the back of the neck at the first prongs on the dogs neck after the chain.