मूढग्राहेणात्मनो यत्पीडया क्रियते तपः।परस्योत्सादनार्थं वा तत्तामसमुदाहृतम्॥
mūḍha-grāheṇātmano yat pīḍayā kriyate tapaḥparasyotsādanārthaṃ vā tat tāmasam udāhṛtam
Word by word
The verse says
Discipline taken up out of a confused stubbornness, with pain inflicted on yourself, or in order to bring somebody else down - that one is dark.
In plain words
Three things, and the third is unexpected.
Stubbornness that has stopped thinking. Self-punishment. And discipline undertaken to damage somebody else.
That last one is worth sitting with. A person can take up a hard practice entirely as a weapon, and from outside it looks exactly like devotion.
Where you've felt this · in health
Your father's brother is deep in a long family quarrel, and he stops eating at gatherings. He says it is for his health. Everybody understands immediately that it is aimed at his sister-in-law, who cooks. He keeps it up for two years. He is genuinely eating less and he is genuinely worse for it by the end, and the point was never the food.
Going deeper
Discipline used as a weapon is the third item here, and it is the one most people have never thought about.
The first two are familiar. Stubbornness with the thinking switched off, and self-punishment dressed up as rigour. The third is stranger: hard practice taken up specifically to harm somebody. Fasting that is really an accusation. Silence that is really a punishment.
What makes it dark in the sense this chapter means is not the cruelty alone. It is that the person has usually lost track of what they are doing. Ask them and they give you the health reason, or the principle, and they are not lying. They genuinely cannot see it.
And that is the warning for anybody, not only for the unkind. Any practice quietly aimed at a person rather than at something in yourself has stopped being practice, and the surest sign is that it becomes harder to give up the better things get.