Kayendriyasiddhih asuddhiksayat tapasah
Kaya = body; indriya = senses; siddhih = perfection; asuddhi = impurity; ksayat = from destruction; tapasah = austerities penance.
Perfection Of (Mastery Over) Body And Senses By Burning Impurities Through Tapas.
The more gold burns in fire, the more purified it gets. The finest steel is that which passes through hottest fire.
Tapas is that fire which makes human beings perfect. Tapas in the form of fast, pranayama, sense-control, kriyas etc not only builds the stamina of body but also that of mind. Due to proper flow of prana in the system, one gets mastery over the body and sense organs, that is, they function in a better way. For example, kriyas like Sankha Prakshalana, wash away all the toxins from the body.
As we will see while discussing prana and pranayama, all the impurities of body can be attributed to improper flow of prana. Pranayama practices like Nadi Suddhi purify the prana and bring a harmonious flow in the instrument that we call body.