Each one of us has done a lot of karma in our previous lives. This karma is apparently done by our gross (physical) body. However, the gross body being inert, it cannot function by itself.

Therefore the scriptures posit a set of forces inside the body but different from it, which is responsible for the activities of the gross body. There are three sets of such forces inside the body:

1). Corresponding to motor functions of the body : Tongue (Speech), Hands, Feet, Genital, Anus, known as the five Karma-Indriyas.

2). Mind : Which includes the five organs of knowledge (Jnana-Indriyas) - Sight, Hearing, Taste, Touch and Smell.

3). The five aspects of respiration (life-breath), namely Prana, Apana, Samana, Udana and Vyana.

These three together are known as the subtle body (Sukshma Sharira). Thus the subtle body is responsible for the activities of the gross body. As this subtle body goes on causing action, there is a continuous reaction on itself – just as a motor, which causes a fan to rotate, gets heated up as a reaction. These reactions take place for each and every action, small or big or trivial, good or bad or neutral; and these reactions go on accumulating every moment.

Theory of Karma - BOOK

So much so for karma when a person is active; finally, when the moment of death approaches, the person lies down and all his activities cease. The first cessation of activities is that of the karma-indriyas; for example the speech. Thus we see that for a person on his deathbed, the speech is the first faculty to go.

At this moment, the accumulated reactions due to our karmas take up a certain shape and get frozen in those shapes. Such shapes are called vrittis; for example, the ultimate shape taken by the faculty of speech is known as vak-vritti. As soon as the vak-vritti gets frozen, the person can no longer speak. He will not be able to tell what happens to him.

This vak-vritti later merges into the mind. Then people around the dying person say: ‘His speech has stopped; nevertheless, he still understands things. He can recognize us.’ It implies that his mind is still working, and expressing itself through the jnana-indriyas. After sometime, the mind, which already contains the vak-vritti, also takes up a final shape, along with the jnana-indriyas.

This shape also depends upon the previously accumulated reactions. Then the mind (along with the vak-vritti), gets frozen into this shape. This final frozen shape of the mind is called the manovritti. After some time, this manovritti merges into prana. Then people around him say that he has stopped even recognizing them, but is still breathing.

They then bring the sacred water of river Ganga and pour a bit of it into his mouth. He is able to swallow it because swallowing is an act of prana (life-breath), which is still functional. Later, in due course, the prana too, which already contains the vak-vritti and manovritti, takes up a final shape and gets frozen into it. This is called the prana-vritti.

An Anthology of Writings on the Ganga (Goddess and River in History, Culture, and Society) - BOOK

This prana-vritti finally enters into the jiva-atman (individual soul); then the people around him say, ‘He is still alive; there is still warmth in his body.’ Then the jiva-atman, who has within himself the prana-vritti, manovritti and vak-vritti, covers himself with this heat and gets out of the body through any of the nine openings. Then the body dies and becomes cold.

These three vrittis form the blueprint for our forthcoming body. For instance, depending upon the vrittis, there could be a spectrum of states in the next birth: Vak-vritti decides whether we will be dumb or stammering or a normal speaker or an orator or a silent person.

Depending upon the manovritti, one can be schizophrenic or dull-headed or normally intelligent or capable of high concentration, or happy or unhappy. Similarly, depending upon one’s prana-vritti, one can be a woman or a man or a eunuch, or sick, healthy, fat, slim and so on.

As for the jiva-atman carrying these vrittis, if during his lifetime the individual had performed some special acts of merit (punya) or demerit (papa), then the jiva-atman would proceed to heaven or hell. After spending his special karma-phala there, he comes back to the earth.

Karma and Karmaphal Mimamsa - BOOK

However, if the jiva-atman has not performed any exceptional karma, then he will come back straight to the earth and not at all go to the intervening heaven or hell. Anyhow, while coming to the earth, he enters into rain and through it into food grains. Here comes the role of God. Circumstances have to be set into place for the jiva to be able to enter into the appropriate father.

The conditions however may not yet be suitable for the father to have him right away. In the meantime, it may so happen that an animal consumes the particular food grain containing the jiva. But after digestion, the jiva will again come out of the animal’s body through waste discharge and then re-enter the food grain.

This shunting will go on till the appropriate father is ready to receive the jiva. When that particular food grain is consumed by the father and digested, the jiva is not thrown out again. Rather, it gets into the father’s seed and from there enters into the mother’s womb, soon to be reborn again.

Karma and Rebirth in Hindu Astrology (Explained Illustratively With Many Horoscopes) - BOOK

This article is based almost entirely on the teachings of Param Pujya Swami Paramanand Bharati Ji. However, any errors are entirely the author's own.

References & Further Reading:

• Bharati, Swami Paramananda. Foundations of Dharma: Bangalore, 2008.
• Shankaracharya, Shri. Commentary on the Brahma Sutras.