The dog doesn’t like to be leashed, pulled back and forth, and held back.
He can not escape if he run forward, so he tries it backwards. In this case a ‘Y’ or ‘I’ belt could be really useful, but this is not the end of the game. Try to imagine the situation with a collar. As the dog is moving and arrives next to the handler, the leash is shortened and tight, and the dog can pull his head off of the collar or the harness. Actually the owner pulls the harness off himself, so he helps the dog to escape without noticing it.
If your dog has a harness, you can use the following method. Put the harness on the dog, pull the strap tight – the best is if there is room for one finger between the dog’s body and the harness, but no more. Let’s start walking, and if the dog starts to go backwards to escape, pull the leash and say ‘no’. In the next moment let the leash to be loose, and treat your dog. If the dog starts to panic, he can not escape backwards if you don’t pull the leash. You can control your dog with small tugs perfectly. With a dominant or panicked dog use your voice to calm him down and show the right direction. It’s obvious that with a shy dog the best method is the calming voice and some treats, but sometimes it can be helpful to work with a dog trainer on double leash: one on the collar, one on the harness.