I don't want to diminish your faith in your trainer, I'd just like you to keep an open mind, which I think you are. I wouldn't describe a prong collar as painful, but rather a clear message. The dog controls the pressure. If they lean or lunge into it, they learn quickly it was their action not yours. Yes you can pop it to get their attention, but 99% of the time, they don't need that. A strong dog needs clear, concise, no-nonsense messages to quickly understand the boundaries of what is OK. An unsure dog who is fearful needs this as much or MORE than an aggressive dog. Fearful dogs need to feel safe and secure. It sounds like an oxymoron to say a correction makes them feel safe, but it does. They finally understand that they are not in charge of their safety, YOU are. They can look to you to take charge and that means they can relinquish the burden of making decisions. I can't tell you how much relief this is to a fearful dog once they get it. Most fearful dogs are aggressive because that's how dogs react, especially when they have to face something (traffic, noise, strange people, dogs etc.) with no choice to run away. You have taken them out on a leash, they can't escape, they have to face these things - so they show fear/aggression then fear/aggression and all they really want is for you to take charge. "Come on, let's go, that car is nothing." Pop the leash and lead him on. "Come on, let's go, that dog staring at you is nothing, and if it comes our way, I'll take care of it. Leave it to me". Pop the leash and lead him on. That is leadership whether your dog is fearful or aggressive or a combination of both, you show your leadership so he has somebody to look up to and rely on.