The experiences that occur in a breathwork session have been extensively documented throughout the last several decades.  Stan Grof created a map of the psyche that included several domains.  This map is often explored in breathwork sessions. Below are the different realms that can often be the charted territory during a session:

 
Realm 1
Stan Grof identified four Basic Perinatal Matrices (BPM) that consist of memories before birth.  BPM I consists of memories before the onset of delivery.  This can include the mother’s relationship and treatment of her body during the pregnancy, her relationship with the baby’s father and other family members and friends, and any traumatic events that occurred during the pregnancy.  BPM II consists of memories during the first stage of delivery when the uterus contracts, but the cervix is not yet open.  BPM III consists of memories during the struggle to be born after the uterine cervix dilates.  BPM IV consists of the memories related to emerging into the world, the birth itself.
 
Realm 2
This realm includes all of the experiences that have shaped our lives after birth including the quality of nursing, the nature of toilet training, traumatic events, access to food, shelter, and water, parental involvement, and other interpersonal relationships.  These influences, especially traumas, leave a particular imprint in the body.
 
Realm 3
Stan Grof used the term transpersonal to include experiences in which consciousness transcends the boundaries of the body and ego and the usual limitations of linear time and three-dimensional space.  This results in experiential identification with other people, groups of people, other life forms, and elements of the inorganic world.  Transcendence of time provides experiential access to ancestral, racial, collective, and karmic memories.  It also includes experiences with the collective unconscious or archetypal figures.  This can include a connection with mythological figures, themes, and realms of all cultures including those of which we have no intellectual knowledge.  In its farthest reaches, individual consciousness can identify with Cosmic Consciousness, the creative principle of the universe.
 
Realm 4 
Stan Grof also discovered during years of facilitating breathwork sessions that faster breathing extended for a long period of time changes the chemistry of the organism in such a way that blocked physical and emotional energies associated with various traumatic memories are released and become available for peripheral discharge and processing.  This makes it possible for the previously repressed content of these memories to emerge into consciousness and be integrated.  Physical manifestations that develop during breathing in various areas of the body are not simple physiological reactions to hyperventilation.  They have a complex psychosomatic structure and usually have specific psychological meaning for the individuals involved.  The tensions that we carry in our body can be released in two different ways.  The first of them involves catharsis.  It is the discharge of pent-up physical energies through tremors, twitches, various movements, coughing, gagging, and vomiting.  It can also include the release of blocked emotions through crying, screaming, and other types of vocal expression.  The second way that tensions can be released is through the use of breathing techniques.  The deep tensions surface in the form of unrelenting muscular contractions of various duration called tetany.  By sustaining these muscular tensions for extended periods of time, the breathers consume large amounts of previously pent-up energy and simplify the functioning of their bodies by disposing of them.