It is typically activated when a person believes he or she has been deliberately provoked. Even when nothing really appears to be provoking them, these people are feeling incited, taken advantage of, belittled, or abused in some way.
Some people find it very hard to express their anger.
Unexpressed anger keeps a person aroused physiologically which can lead to health problems like high blood pressure and even heart disease. Also, unexpressed anger can cause feelings of helplessness, which can, in turn precipitate depressed mood. A far more common problem is that of people exaggerating the provocation in situations, particularly interpersonal ones, such that they feel intense and prolonged anger unnecessarily. All of these techniques can be helpful for some people. I believe that though doing such things feels good, it doesn't do anything to reduce what it is that is bothering you.