In shamanism it is believed that part of the human soul is free to leave the body. The soul is the axis mundi, the center of the shamanic healing arts. Shamans change their state of consciousness allowing their free soul to travel and retrieve ancient wisdom and lost power.
Because a portion of the soul is free to leave the body, or it will leave the body to protect itself from potentially damaging situations be they emotional or physical. In situations of trauma the soul piece may not return to the body on its own, and a shaman must intervene and return the soul essence.