कर्षयन्तः शरीरस्थं भूतग्राममचेतसः।मां चैवान्तःशरीरस्थं तान्विद्ध्यासुरनिश्चयान्॥
karṣayantaḥ śarīra-sthaṃ bhūta-grāmam acetasaḥmāṃ caivāntaḥ śarīra-sthaṃ tān viddhy āsura-niścayān
Word by word
The verse says
They wear down the gathered elements inside the body - and me, who am sitting inside that body too. Know that their resolve has gone over to the demonic.
In plain words
Here is the line that makes the previous one serious.
When you grind yourself down, you are not only damaging yourself. Krishna says: you are doing it to me, because I am in there.
That is the same fragment named a chapter and a half ago. Harshness toward your own body is being counted as harshness toward him.
Where you've felt this · in health
You skip meals when work gets heavy, and you are a little proud of it. You would never let anybody who works for you do that. If one of them said they had not eaten since morning you would send them out for food and mean it. The standard you keep for everyone else has one exception written into it, and the exception is you.
Going deeper
Harshness aimed at yourself is counted here as harshness aimed at the divine, and the ground for that was laid earlier.
Chapter 15 said a fragment of the divine is seated in every living thing. That includes the body you are currently being hard on. So the self-punishment is not a private arrangement between you and you. There is somebody else in the room.
This cuts against a long habit, here and everywhere else, of treating cruelty to oneself as the safe kind of cruelty - admirable, even, because nobody else is hurt by it. The verse simply refuses that.
Which gives you a usable rule, and it is not a soft one. The standard you apply to the people you are responsible for is the standard. If you would not let somebody you care about treat themselves the way you treat yourself, you already know what this verse is about.