The are a fabulous training aid for those who know how to use them and like any other training aid can be abused by those who do not know how to use them.
Before use the dog must understand the command being given. The collar is fitted and the dog allowed to relax and when the dog is calm and relaxed the dial which is set at "0" is gradually and slowly increased until the dog reacts, by reacts I mean say a flick of its ear, that is the lowest setting for that particular dog and the setting you want to use.
Some people then use a constant stimulation and when the dog complies with a "known command" stop stimulating the dog, in essence the dog learns how to switch off the stimulation, the faster he sits the quicker the annoyance round his kneck stops.
In situations where the dog is reckless you can turn the dial up to an uncomfortable setting with the dog quickly learning that it should cease doing whatever it is doing, this of course is where abuse can take place. The dog that doesnt know a command, or an impatient handler, can quickly abuse his own dog causing real problems with the training thereafter.
I have seen stock chasing dogs stopped dead in their tracks and not want to look at another sheep, the alternative "Shoot the dog?" as always some people, (including me until I was taught how to use one) think they are cruel, so it becomes an emotive issue and the cause of major rows.