Reincarnation in short
Reincarnation in The Spiritual Theory (ST) is a natural phenomena cause by the great affinity between the Spirit and the spiritual components in the forming body.
Incarnation and reincarnation is the temporal union between the spirit and the physical body, specifically, the brain. When the human embryo is forming, it sends a spiritual signal, a spiritual vibration into the spiritual dimension (which is different and exist separated from the material dimension) to attract the most affine spiritual being to that body. At this moment, the spirit experience two phenomena: the state of confusion, which submerge the spirit into a state similar to that one we experience when we are asleep; and forgetfulness of the past, which block all his past memories leaving the incarnated spirit in some kind of stupor state until a new set of memories related to his physical experience are acquired. That allows him to awake from this state as a conscious human being.
During incarnation the spirit does not move into the physical reality, he actually remains in his own spiritual reality, but he is firmly integrated with the spiritual components of the brain through which he lives the physical experience. When the brain dies the strong and complex interconnection between the spirit and the brain disappear and the spirit is slowly release from incarnation, recovering his past memories and awakening from the confusion state. It is the same being we knew as a person, since the body is just a vehicle of the spiritual expression.
Back into the spiritual reality, the spirit shows a state of harmony which reflects the progress or not acquired during the physical experience. If there were small or no progress, he surely retain a high level of affinity with the present physical and social spiritual structure of the planet, increasing the possibility to be attracted and incarnated in a short period of time. This is one of the reasons why the average time between lives in children with memory of past life is only 12 years, according with data collected by Dr. Bruce Grayson.
However, if the spirit had a life of progress increasing his level of harmony, then he probably will not be affine with the present physical structures (people) but with others in the distant future, enlarging the time for reincarnation. In this sense, reincarnation will continue over and over, until the spirit reach such an elevated state of harmony that he no longer has any compatible physical structure to incarnate, continuing his path of progress at the spiritual level only.
The spirit is a real entity whose existence was not only verified by mediums, but also by Near Death Experience patients. The spirit is not something that we human beings have; the spirit is us, the spirit is ourselves thinking and talking through the physical body. We are the conscious beings, we are the conscious firmly attached to the brain and the one that can witness what is happening around the body during NDE. We are the same beings that after the death of the brain continue existing in the spiritual dimension and can comeback sometime in the future to continue this process of physical experiences and progress in search of an enlightened state.