I'm not a big fan of using "pain" when I do the forced retrieve. In fact, last year, Greg had to do a forced retrieve with TJ - the dog that I was showing. Why did HE have to do it? I couldn't bear it. We had to do it in a very short period of time, and using pain is the best method to do that...Did it work? Usually - as long as there wasn't a decoy around, in which case he decided to bite anyway!
Greg used the e-collar at a high level with TJ.
I also know that Cy was trained with the forced retrieve using pain - specifically the ear pinch method. I don't know exactly how Todd did it, but many people take bottle caps (the metal ones) from bottles and use those to pinch a dog's ear. When the dog takes the object and holds it, you let up on the pinch.
Jerry prefers to backtie a dog on a flat collar, then use a pinch collar. That way the corrections are coming from you. Once you take the dog off the back-tie, you use a lighter correction to force the dog towards the object, and you keep tugging. When the dog picks up the object, you run backwards to get the dog to bring it to you quickly. Eventually, you do transition to the e-collar at a low level.
Even though Cy was trained with the ear pinch and would have a nice retrieve on flat land with no objects or decoys around, it was a different story when we had to throw the object in something and there was a decoy taking off the other way at the same time I sent Cy to retrieve. So, since he already knew the command, Jerry just had me tug Cy towards the object, then run backwards when he got it...now we use the e-collar at a low level.
So, how does the dog know you are using it as a motivator and not a correction. There are several factors. The first is simply how it's introduced. With Cy, we paired the tugs on the pinch with the e-collar stim. He already understood the pinch tugs, so we just added in the e-collar stims. Second is the level that you're using on the e-collar. My e-collar has setting from 0 up to 127. In obedience, I typically have to correct Cy using a level of 23. During bitework (when he's working in a much higher level of drive) I usually have to use about 45. When I use the e-collar as a motivator, I am using it at a level of 12. So, what it becomes is a tap on the shoulder (like "hey you, remember me, you have to get that object and bring it back.") As soon as he is on his way back to me, I stop stimming him. Because it's a lower level (about 1/2) than his normal corrections, he figures out, oh, she's not correcting me, just nagging me...to get it to stop, he does what I'm telling him to.