It believes that the past, present and the future lives of all humans were foreseen by Hindu sages in ancient times. Nadi astrology is based on this.
According to Hindu faith, thousands of years ago sages who had the power to look in the past and future of the entire universe recorded the future of people who were living and who are going to live, in the form of Tamil verses, as Palm Leaf Manuscripts. There are different schools of thought as to the author of these leaves. They were said to be written down by different Hindu sages, who had divine revelations. Some of the sages include Agasthiya, Brighu, Kaushika, Vyasa, Valmiki, and Vasishtha.
Usually, the Siva Nadi Jyothisham astrologer asks for the thumb impression (right hand for males and left hand for females). The Nadi Jyothisham astrologer then searches his repository of leaves for the seeker's classification of the thumbprint. Finally, the minimum possible set of matching leaves is brought. Every leaf corresponds to some individual and hence will bear the birth and kinship details of its seeker. Once the exact leaf is found, the Siva astrology tells the seeker's name, parents name, spouse name and many other details about his future, since they are all written in the leaf.
It is very powerful and accurate. There is no prediction involved - only translation/interpretation of what is written on the leaf.