The next step in training dogs with a head collar is to train them that when they reach the end of the leash they are going nowhere. That means the owner must hold perfectly still and avoid taking a step or even moving the leash-holding hand. Once the dog figures out that pulling harder does not work and instead steps back or turns to the owner such that the leash is hanging loose, then the owner can resume walking. Better yet, the owner can reward the dog with a treat so that dog comes all the way back to her and then they can resume walking forward. It's important that the dog learn that a tight leash and the associated pressure created means she should stop. If the dog is not taught this and tends to act impulsively, she may dart out after a cat or other object and hit the end of the leash with some speed. This type of accident could potentially cause neck pain or injury. Even in the emergency situation, if the owner is paying attention, they can prevent neck wrenching if they gradually tighten the leash rather than letting the dog dart forward on a loose leash so that she suddenly hits the end.
So there you have it, an overview of a variety of common collars and harnesses. None is perfect. They are all just tools. But some are more likely to cause problems in your pet or may just provide a less than ideal match for your needs. In case you're wondering which I prefer—ideally my dog can walk on leash with the flat collar he wears regularly. But for those dogs that tend to pull and need more work, I tend to recommend a front-attaching harness or a head collar of some sort.