What is Thamasika Yoga?
Thamasika yoga = Thamasic Saadhana
The practices under this saadhana are totally opposed to higher order and instead take the practitioner to the darker realms of creation.
These practices include invoking of dark forces of nature (spirits, goblins and other lower forces) through violent and unnatural ways of worship including animal sacrifices etc., only to gain control of those dark forces of nature and use them against fellow human beings by way of controlling, exploiting and misleading them, as a super human being for personal glory, name, fame and riches.
This appeasement of dark forces of nature through these practices and misusing them, makes the practitioner to accumulate more and more karma and by sheer weight of karma, he goes down to the lower and darker layers of creation (animals and plants etc.), just as heavy things drown in water.
This is what Lord Krushna means when he says “Adhoo gachanthi Thamasaaha”. The Raajasic saadhana and the Thamasic saadhana are not the path of Salvation/Moksha, like the Saathwic Saadhana.
Note : The spiritual practices which are not natural in their practise (unnatural) are not true spiritual practices.
Spiritual practices cannot be against the natural way of life.