अशास्त्रविहितं घोरं तप्यन्ते ये तपो जनाः।दम्भाहङ्कारसंयुक्ताः कामरागबलान्विताः॥
aśāstra-vihitaṃ ghoraṃ tapyante ye tapo janāḥdambhāhankāra-sanyuktāḥ kāma-rāga-balānvitāḥ
Word by word
The verse says
Some people put themselves through terrible discipline that no teaching ever asked for, full of show and self-importance, pushed along by wanting and by the force of attachment.
In plain words
A warning about extremes, and about who they are really for.
Notice what is not objected to. Hard practice is not the problem - a whole section of this chapter is about it, approvingly.
The problem is three words: show, self-importance, and force. The practice is severe because severity is being watched, including by the person doing it.
Where you've felt this · in sport
A boy at your gym trains through an injured shoulder because stopping would mean admitting something. He tells everyone it is fine. He is in real pain and will not say so, and after four months the shoulder is worse than any amount of rest would have left it. Nobody made him do this. He could not work out how to stop without it meaning something about him.
Going deeper
Severity being admired by the people watching is what makes this verse necessary, and the trap is older than any of us.
The objection is not to difficulty. Hard practice has a good name in this book and gets three admiring verses shortly. The objection is to difficulty chosen because it looks like something. Three things are named: show, self-importance, and the pressure of wanting.
There is a reliable way to tell the two apart, and it has nothing to do with how hard the thing is. Discipline that is working makes a person easier to be around. Discipline that has gone wrong makes them harder, and usually louder about it.
The second test is quieter. Would you keep it up if nobody ever found out? Most of what belongs in this verse does not survive that question, and most genuine practice does not even notice it.