The reason dogs pull on the leash is simple... because they can. They pull, we keep walking; they pull harder, we walk faster. A controlled walk is not a formal "heel"; it simply means that you can take your dog for a leisurely walk on a slack leash and be able to divert his attention from things he may find appealing when you need to. For a dog to learn a controlled walk, we need to teach him that good things happen when he is not pulling on the leash. Pulling, yanking or "popping" the leash may be a momentary fix, but not only can it harm your dog, it is an intermittent occurrence, so the pulling behavior will only reoccur the next time your dog wants to sniff something out of its reach or chase after the occasional squirrel. You can begin working on a controlled walk by simply walking around your home.
Use a phrase like "let's go" to tell the dog when you are ready to start and/or change direction. This gives him an opportunity to respond without pulling on the leash.
We take our dogs on walks to relieve themselves, to exercise and to enjoy being outdoors experiencing different sights and smells. It is natural that they want to sniff and explore, but it is recommended that to continue to reduce the chance of pulling, you give permission at a location of your choice to allow the dog to sniff and/or relieve himself. Say a phrase such as "go sniff", "at ease", or "go play" to signal to your dog that it is okay to stop and sniff that spot. When you are ready to begin walking again, say your strike off phrase "let's go" and continue on your relaxed walk.