It's common for a dog to pull against the pressure of a leash attached to their back or the back of their neck, and as far as i know, they'll do it with either a collar or a leash. Zack sure did. He loved to pull.
At first i was using a collar because i didn't know any better, about the vulnerability, and he pulled hard. Then i learned that this can potentially harm a small dog, and i got a harness that had the clip on the back, and he pulled as hard as ever, but i didn't worry as much because it wasn't choking his neck, although it still seemed like a lot of strain on his body as he would pull me along.
Then i had him at a puppy class and another dog had an interesting looking harness on and i asked about it and it was an Easy Walk harness, designed to reduce or eliminate pulling because the clip is on the chest--it's not fun to pull when it's clipped to the chest, the way it's fun when clipped to the back. Pulling when it's attached to the chest tends to throw them off balance a little, and from the first day i put one on Zack, the hard pulling stopped, just like that. There were times when he would pull on it when excited by something, but he would never pull for long. Before i got that harness, he pulled constantly. Now, he rarely pulls and never has hard as he used to.
He's gotten out of the habit of pulling now. I have a Puppia and he walks fine in that. He pulls a bit more in it, they love that feeling of pulling against something attached to the back of them, but he's not big on pulling anymore. He's trained to heel on command.
Anyway, i've had a really good experience with the Easy Walk and will definitely recommend it to eliminate pulling.
here's what it looks like
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Here's zack on a retractable leash with the Easy Walk, you can see the leash isn't taut. It's a little slack, even though he's in a forward mode.