Bckrazy I think you might be on to something nobody has thought of before.
I think some dogs might associate the correction with when it was corrected, for example pulling to get to someone or something.
Out trainer uses these for extrememly hard dogs so I can't wait to discuss this with her.

This triggered a memory from long ago. My friend went riding and her Sheltie tagged along, For some reason the dog screamed out in pain and my girlfriend thought the horse might have stepped on it's toes. But when she returned home the dog was limping badly and wouldn't go near her or the horse. She had it vetted and found out it was the kneecap that had turned or something, had nothing to do with the horse, it just happens with shelties. To this day, the Sheltie refuses to go near the horse.

What I don't understand is if the dog has been chosen for service work, why would someone train it with a prong? I've never seen any SD trained with a prong, facilities just don't use them.