The DSM describes main feature of dissociation as a disruption of memory, consciousness and identity or perception. Dissociation is a protective mechanism. Human beings have at their disposal to survive traumatic events.
Abusive painful experiences and memories are put away into isolated compartments in the mind, and separated from regular memories.
Read also: Traumatic and Non-Traumatic Memories
Dissociation is a way the mind organizes information
Dissociation refers to a compartmentalization of experience: elements of an experience are not integrated into a unitary whole but are stored in isolated fragments (van der Hart et.al., 1998).
Exposed to trauma, the mind splits. The part of the brain that continues with the daily functioning of life (the left brain), and the emotional part of the self that holds the traumatic memories (the right brain) and its survival impulses of the moment of trauma becomes unintegrated with each other. This leaves the person with a split sense of self.

Experiencing a split sense of self can be disturbing. To notice the phenomenon as it happens is to gain agency.
The disruption of integration of the hemispheres of the brain leads to the experience of feeling something and “not making sense” of the feelings. The feelings come in the form of emotions, perceptions or physical pain.
The experience of not making sense of what one feels, can be disturbing. As human beings we need to make meaning and understand things about ourselves to feel safe. When such splitting occurs, the trauma survivor experiences blankness and confusion. This contributes to more insecurity. Oftentimes the need to make meaning results in thoughts that are paranoid in nature, intrusive and/or obsessive.
In therapy, clients are guided to 1. first identify the feelings and sensations that make no sense, 2. accept these feelings without making meaning. 3. observe the nature of thoughts that arise from attempt to make meaning, and 4. allowing these sensations to pass (through relaxation or somatic exercises). Each of these steps are tedious and challenging, needing full attention of both therapist and client. This is also solid mindfulness work. The result is the client gaining of agency of the self.
Splitting leaves the client fragmented into parts of personality. As different times the person’s right brain may trigger experience in him/herself a part that is raging and wants to fight or take revenge, a part that is terrified, a part that is ashamed, a part that is needy and/or a part that wants to run away. When these parts are traumatized, they feel out of control.
The left brain engages the other parts of the person that wants agency. These parts manage daily function, the part that is sociable, and the part that is responsible.
Noticing split-off and traumatized parts
One can notice that splitting has occurred through phenomena like experiencing chronic inability to make decisions, continually relapsing into addictive behavior, having intrusive emotions that seem to arise out of nowhere, intrusive thoughts, shifts in mood or behavior, going numb, getting hyper-aroused, collapsing, feeling suicidal, hearing voices, loss of ability to connect with others, difficulty communicating, withdrawal from society, feelings in the body and somatic symptoms that are not based on medical logic.
There are different severity levels of dissociation
Dissociative symptoms can be severe in some people to a point of rendering them incapacitated. Many individuals, however, experience dissociative symptoms, and are still able function and be successful in life.
Treatment of dissociative symptoms with therapy in functioning individuals is a measure to keep the person healthy and functioning. While we can cope with dissociative symptoms, these symptoms do not disappear on their own. Symptoms get worse with age, and are exacerbated by crises in life. This is why and how some seemingly functioning people experience sudden psychological breakdown.
Signs to look out for in functioning individuals
It is clear that severe dissociative symptoms require professional attention. Less obvious or hidden signs of dissociation are worth noticing: 1. difficulties putting things together, not being able to remember conversations, forgetting appointments, or inability to recount coherently what happened in certain situations, 2. experience of doing things that does not seem to add up, like having sexual relationship with someone one finds unattractive, 3. having unexplained chronic pain or somatic symptoms, 4. chronic experience of stuck in life, 5. experience of identity confusion, 5. experience of self-harming or suicidal thoughts.
Therapy that focusses on mindful observation of these symptoms, its triggers and the trauma underlying lead to successful outcome in providing clients with agency over his/her life.
Bibliography
van der Hart, O., van der Kolk, B. A., & Boon, S. (1998). Treatment of dissociative disorders.