18.33 O Partha, dhrtya, the firmness; (-is connected with the remote word) avyabhicarinya, that is unfailing; yogena, through concentration, i.e. (the firmness that is) ever associated with samadhi (absorption in Brahman); yaya, with which; dharayate, one restrains;-what?-manah-prana-indriya-kriyah, the functions of the mind, vital forces and organs-restrains them from tending towards the path opposed to the scriptures-. Indeed, when restrained with firmness, they do not incline towards objects prohibited by the scriptures. Sa, that; dhrtih, firmness, which is of this kind; is sattviki, born of sattva.
What is mean is that when one restrains the functions of the mind, vital forces and organs with unfailing firmness, one does so through yoga, concentration.