Martingale collars (also called limited-slip collars) are usually made of flat nylon with a smaller fixed-length section (made of either nylon or a short length of chain) that, when pulled on by the leash, shortens up tightening the collar around the dog's neck, to a limited extent. When properly fitted, martingales are looser than flat-buckle collars when not tightened, and less severely corrective than slip collars when tightened.
Every dog needs to learn to walk on a leash. Besides the fact that most areas have leash laws, there will be times when keeping your dog on a leash is for his own safety. Learn how to introduce your dog or puppy to the leash, then teach him how to walk properly on the leash, even beside you on a bike. A loose leash walk teaches your dog not to pull or lunge when on the leash, making the experience more enjoyable for both you and your dog.