The great yogis carefully protect their self illuminating atman like a pregnant woman protecting her child in the womb. From whom the sun is rising and in whom the sun is setting, and from whom all gods originate and depend is the atman that Naciketa is seeking. You know that Brahman is situated in your heart chakra and you also know His supreme value. There is nothing more valuable than him in this universe. Knowing well that you are possessing the most valuable thing in this universe, you are bound to keep it carefully. That is why Yama says that yogis protect the atman with great care. The one who does not realise the universal existence of the Brahman suffers from births and deaths. There is no necessity for you to search the Brahman outside your body. It is not necessary that you should seek Him in a church, mosque or temple. He is there within you, therefore seek Him within. What is the meaning for keeping the Brahman carefully? Yes, you know well that Brahman exists everywhere. Since you also know that he is within you as well, you have to keep your mind and body clean so that the place in which the most valuable thing in this universe is placed (your mind and body) is kept clean. When you know that Brahman is within you, you should not give room for bad thoughts and rash actions. These things do not keep your mind and body clean. This statement of Yama underlines the importance of Brahman. Yama not only talks about the Brahman but also talks about its surroundings. Yama now proceeds to the description of the Brahman. He is in the cave of the heart in the form of light without smoke and in the size of our thumb. There is some variation in the size of the Brahman though there is no difference in his location. Most of the scriptures describe the Brahman as smaller than the smallest. But Yama says that it is in the size of a thumb.
My personal interpretation is that the reference to the size of the thumb means the cave and not the atman. Within this cave there is soul and within the soul there is atman. Naturally he has to be the smallest of all and his size cannot be compared to anything. There is yet another explanation for this. Yogis enlarged the size of the Brahman like enlarging a photograph. Since smaller than the smallest cannot be meditated upon by ordinary men, they have enlarged the size of the Brahman enabling us to meditate upon him. Meditating on shunya (nothing) is based on this fact only. The one, who realizes the Brahman, becomes Brahman himself like handful of water when poured into the ocean becomes ocean itself. The abode of the Supreme Self is our gross body and this has eleven openings. This supreme Self which is without changes and always remaining the same is being serviced by these eleven openings in our body. These openings are two eyes, two ears, two nostrils, mouth, naval, organ of procreation, organ of excretion and a small opening (brahmarandra) in the in the crown chakra. The soul of a yogi leaves his body through this orifice at the time of the physical death of his body. These eleven openings are used in servicing the Brahman. The body is compared to a city and these eleven opening are said to be entry points to this city. The whole city is controlled by the Brahman. We have to distinguish between the ruler and ruled. Ruler is the Brahman and ruled is the body. The one who understand this concept – the atman or Brahman or the Supreme Self is different from the body, gets away from the clutches of birth and death. Naciketa questioned Yama only about this Supreme Self or atman. The one, who understands this, wins over the death itself. Death is for the physical body alone and not for the Brahman as he is beyond birth and death.