Some folks gets confused about positive punishment (adding something unpleasant when the dog has done something unwanted) and negative reinforcement (something unpleasant to the dog is already happening, and is taken away at the moment the dog does something that is wanted).
Let's take the example of leash corrections. Some people use lots of light leash corrections when teaching their dog to heel. Shock collars operate on the same principle (only difference is that people's arms don't get tired from giving constant corrections.) The dog will learn to stay in the proper position in order to avoid having his neck jerked around or being shocked. That is actually negative reinforcement. It is also called avoidance, or escape.
If the dog is walking nicely, sees a cat and lunges, at which point the leash is jerked or the dog is given a shock, then that is an example of positive punishment. Nothing unpleasant was happening to the dog until he lunged and only then was the leash yanked or shock given.
The problem with negative reinforcement is that in effect the dog is being made to feel uncomfortable BEFORE he has even had the opportunity to do anything at all. If the dog is still at the beginning stages of training and is uncertain how to perform, then theoretically this unpleasantness could last a long time, or even forever. Being made to feel discomfort for no reason that the dog can see or understand, can lead to learned helplessness. That is the major argument against the use of this type of training.
So for an easy way to spot the difference, folks have to ask themselves what was the dog experiencing BEFORE the dog did the action, and before the command was given. Then it becomes very clear if the leash corrections or shocks are positive punishment, or negative reinforcement.
Hope this helps!