Tapas
tapasDiscipline, austerity, or "heat": the third niyama, invoked whenever a class gets hard on purpose.
The root sense of tapas is heat: the friction of doing the thing you said you would do. The Sutras state its result plainly:
Perfection in the body and in the organs after impurity has dwindled as a result of self-castigation.
The original Sanskrit
kāyendriyasiddhir aśuddhikṣayāt tapasaḥ
Impurity dwindles, and the body and senses sharpen. Every time a teacher says "stay with it" in a long hold, this is the word standing behind them.