In the first part of training for a dog, before the dog has learned what you want him/her to do, it is tempting to try to hold the dog in the place you want him/her to be — pulling on the leash so there is constant pressure on the dog’s collar. We call it a ‘tight lead.’ That’s the wrong approach.
Instead, leave the leash lip until you have to tug gently on it. This ‘loose lead’ puts the responsibility on the dog to choose where to put himself or herself.
As the trainer you win either way — you can correct the dog or you can praise the dog depending on the position the dog has chosen.
The photo here shows examples of tight and loose leads. Here is a close-up illustration.
In this short video watch the handler work with this Bull Terrier puppy with a loose lead. Occasionally, the handler will tug gently on the leash to get the dog’s attention when the attention wanders off task.