The austerity that is practiced with confused notion, torturing the Self, or to destroy another — that is declared tāmasika.

Tāmasika tapas: mūḍha-grāheṇa — with deluded grasp/notion (mūḍha = delirious). The method: ātmanaḥ pīḍayā — torturing one’s own Self, self-destruction under the name of religion.

Another motivation: parasya utsādana-artham — to destroy (utsādana) the other (parasya). Black magic, curses, austerity to harm. Udāhṛtam — declared as tāmasam. This is the opposite of true tapas.