Abreaction is living through or reliving a traumatic moment in a person’s history so that the emotions attached to it can be re-experienced and released. Early in the history of psychology, the idea of abreaction was developed, and from Sigmund Freud onward, the general contention has been this technique provides a way to free a person of unconscious emotional content that has bound him or her, and that it becomes a way to escape the demons of the past. Today, there are many therapists who utilize abreaction and many who absolutely don’t — much depends on the therapist’s grounding and the types of patients the therapist might treat.
The idea of abreaction is bound up in the concept of catharsis. This is an emotional cleansing or purging of the spirit that often happens second hand through experiences like watching a film or reading a book. Something within the second hand view activates emotional content, conscious or unconscious, and this may cause people to contact their deep emotions and process experiences or emotions vaguely reminiscent of the second hand (reading/ viewing/listening) experience. Indirectly, the person has "gone through" something quite like what he or she once experienced, but there is a slight emotional removal that makes it easier to process.
With therapeutic abreaction, catharsis is its intended goal, in many cases. When undertaken with a therapist, the person does have some removal from the initially provoking situation, which provides a cathartic distance. A "reliving" that occurs without guidance is a different matter. Many people who suffer from post-traumatic stress disorder experience sensations that they are once again living a terribly traumatic moment in their lives. Triggers for these flashbacks are numerous, and a certain sound, smell, temperature, or sight could provoke them. Under most circumstances, a flashback is not an emotional release and instead may only worsen a person's stress, but in therapy, the induction of flashbacks or memories to process trauma is not uncommon.
In therapy, abreaction is generally more direct, and traditional psychoanalysts like Freud and others sought to specifically have people relive a trauma. One of the convenient tools for this was hypnosis, which could help clear away a person's conscious resistance to reliving something difficult. Abreactions could and can be quite dramatic, with people undergoing horrific or just mildly traumatic experiences under hypnosis or with other tools. They tend to provoke extremely strong emotions, and the analyst must be able to help a patient through them, depending on the memory, to lead to a release from the trauma, with additional analysis.
In modern times, one method that has evolved to de-intensify abreaction, but still allow for this re-experiencing, is eye movement desensitization reprocessing (EMDR). Therapists attempt to help reduce the intensity of an experience that is being re-experienced by having clients return to an emotionally safer place. When to do this is the judgment call of the EMDR therapist because part of the point is finding areas of extreme trauma and releasing them, and sometimes despite intent of the therapist, full abreactions occur that are much longer and more intense than planned.
As with just about any psychological concept, there is debate regarding the necessity of "reliving." Some people feel it is unnecessary to promoting wellness and runs the risk of re-traumatizing the client. Others think it’s important for some clients but certainly not others. Where it is most often used is with people who have post-traumatic stress disorder (PTSD). Some methods like EMDR are highly successful with PTSD, although there are people who recover from PTSD without abreaction.