shraddhayaa parayaa taptam tapastattrividham naraihi |
aphalakaanshibhiryuktaihi saattvikam parichakshate || 17 ||

This three fold penance, performed by balanced people with supreme faith, without expectation of reward, is called sattvic.
shraddhayaa : with faith
parayaa : supreme
taptam : is performed
tapaha : penance
tat : this
trividham : three fold
naraihi : by people
aphalakaanshibhihi : without expectation of reward
yuktaihi : balanced
saattvikam : sattvic
parichakshate : is called
So far, we saw that tapas or penance comprises three aspects. Bodily or shaaririka tapas, speech or vaangmaya tapas, and mental or maanasika tapas. Tapas is used to conserve and channel our energy in the pursuit of a goal or objective, whether it be material or spiritual. Shri Krishna now describes three types of tapas, and how they can be used to assess the texture of our faith. He first describes the conditions under which penance is revealed to be sattvic.
The unique nature of sattvic tapas is that it is performed in the pursuit of the highest possible goal, which is self realization. It is not performed for any material gain, or the pursuit of physical or mental powers. Furthermore, it is performed with the utmost faith in the statements of the scriptures. Shri Krishna uses the word naraha or human in this shloka, and not in the shlokas that described the other two types of penance. He implies that only humans have the ability to perform penance without expectation of material gain.
Who has the ability to perform this highest level of penance, this sattvic penance? It is one who is yukta, one who has integrated his mind with his intellect, one who can remain balanced in success and failure. Only such a person is able to incorporate all the three aspects of penance, physical, mental and speech, referred in the shloka as three fold. Even if one of these is missing, the tapas loses its sattvic aspects. Mental penance is the toughest of all the three, since it is hardest to conquer the mind.