The deepest austerity happens where no one can see it — inside the mind. Krishna lists five qualities: serenity of mind, a gentle disposition, inner silence, self-restraint, and purity of intention.
Serenity means staying calm and content from within, regardless of outer circumstances. Gentleness means responding to the world softly, not aggressively. Silence here is not just closing the mouth — it means stilling the mind's restless chatter. Self-restraint means holding the senses and impulses in check.
The most important of the five is bhava-samshudhi — purity of feeling. When the heart is free from jealousy, resentment, and deceit, mental austerity is complete. This inner work is invisible to others, which is precisely what makes it the hardest and most valuable form of tapas.