What is Naadi Jothidam, Naadi Josiyam, Nadi Astrology?

'naadi' means, 'In search of' in Tamil.

Since a person goes 'in search of' his scripture(Olai chuvadi), using his thumb impression, it's called 'Naadi Jothidam'.

First, Thumb impression(right for male, left for female) are taken, and the astrologer identifies the pattern in the impression and identifies the category in which the scripture(Olai chuvadi) will be found.

Olai chuvadis are tied in bunches of 100(much like stapling 100 papers).
Each bunch has 50 Olais(leafs) and it has contents written on both sides, counting to a total of 100.
So each bunch will have information of 100 persons(one leaf(Olai) for each person).

The first chuvadi is like an index.

The astrologer asks few questions, for which you have to respond 'Yes' or 'No' and he eliminates the non-matching index Olai's and finally he might end up in one Olai in which all the questions will end up with getting a 'Yes' from you. At that point, they have identified your 'Olai Chuvadi'.

Then they get the detail scriptures(another bunch of chuvadis) and read it for you.

" Vedaashcha kratavashchaiva kratuunaam phalameva cha .
Yaani krityaani lokeshhu sarva eshha ravih prabhuh"

--Aditya Hrudayam, 24

The Sun is verily the Lord of all action in this universe. He is verily the vedas, the sacrifices mentioned in them and the fruits obtained by performing the sacrifices.