Again, I'll start.
I use it mostly for two things. One is as an attention-getter when he is hell-bent on heading off in his own direction, to remind him that I'm there and he needs to come back in touch with reality.
The other, and more common use, is as feedback. I use a gentle nick (what Dan calls a "nag") to let him know he's made the wrong choice when he's pretty far from me. It's immediate feedback for him that he needs to make a difference choice, which I believe ultimately helps his problem solving skills.
Example....I give him an "over" cast, which he knows by now. He heads back. Gentle "nick", wrong choice buddy. Sit whistle, try again. If he heads the right way, I know he understood my feedback and made a different choice. If he heads the wrong way, the feedback becomes a bit stronger. NO, not THAT direction....
Truthfully I don't use it much right now. I am still a student and learning and I approach its use with caution. Plus I have been told since I am in transition to not use the collar at first while running the blinds so that I do not lose momentum and confidence. Once she knows what she is doing and has some experience under her belt that will be different.
Sometimes I have to use it for 'here' because she has her moments when coming back on the retrieve where she wants to muck around. Sometimes I use it when she wants to slip a whistle sit. I used it in FTP. It helped me get through a tough spot in single t when she was popping and anticipating the whistle sit.
The more skilled and schooled the dogs become, I find the less I use the collar as well. The collar for me gets most used during basics beyond three-handed casting--FTP, TT, Waterforce and swimby. There it is used for violations of basic obedience principles: go, stop, come. Once we move from the basic yardwork into things like 3 legged patterns, I most often use attrition for while they are learning, and the collar is still only used for violations of those obedience elements. Once we start to add concept lessons, it is still attrition, as well as breaking the concept down into simpler pieces if the dog fails to convert and do the new task correctly. Once the dogs are schooled in a concept, however, I will add a collar correction for failure to obey things like casts in the presence of suction factorsor avoidance of a factor that is online, etc. Generally I stop once with attrition and then if the dog persists in doing what they want instead of what they have been taught to do, that choice warrants a correction. But ONLY if I have taught them what is expected, and I am assured that they understand it. My dogs always wear their collars in training, but rarely get a correction as we try to set them up for success, and build sequentially, but sometimes they are determined to take the easy way out, and that is when the correction is warranted.