Once we admit the incarnation of an Avatar, we come to the practical questions of how the physical framework, the body, life and mind of the Avatar are prepared to receive the higher force and intensity that the Avatar brings, and how the descent of the Avatar actually links up with and embodies itself into an appropriate human life.
One potential process is simply that the Avatar takes birth in a body prepared through the normal developmental process of Prakriti. As human development occurs, successively higher intensities can be sustained, leading to the ability of the Avatar to work in a body that is suited to the time within which he is undertaking his action.
Another potential would be for the Avatar to actively bring together the elements and shape the body for his future incarnation.
Sri Aurobindo weighs in on these issues while analyzing the Gita’s statements in this regard: “If we suppose that the body is always created by the hereditary evolution, by inconscient Nature and its immanent Life-spirit without the intervention of the individual soul, the matter becomes simple. A physical and mental body is prepared fit for the divine incarnation by a pure or great heredity and the descending Godhead takes possession of it.”
It is however not so simple. “But the Gita in this very passage applies the doctrine of reincarnation, boldly enough, to the Avatar himself, and in the usual theory of reincarnation the reincarnating soul by its past spiritual and psychological evolution itself determines and in a way prepares its own mental and physical body.”
This would imply an evolving series of Avatar births, with the Avatar preparing the body needed for the birth in the particular age and circumstances in which he is to be born at a particular time.
This is one of the most difficult concepts for the modern mind to grasp and accept, because it involves concepts (or realities) that the modern mind is not comfortable with holding. The materialistic bent of the age finds it difficult to accept that a spiritual principle is the core impetus of existence in the material world, and that consciousness creates the material world, rather than the other way round.
Sri Aurobindo, Essays on the Gita, First Series, Chapter 16, The Process of Avatarhood, pp. 156-157