When used as a tool for self-transformation and awakening to clearer awareness, yoga starts the moment a student first pays attention to what he or she is doing in the practice.
If a student is unsteady, falling, in pain, or distracted by discomfort, the tendency will be to go back into his or her analytical or agitated mind. Sthira and sukham— steadiness and ease— give the asanas their transformative potential.
Being steady does not mean being perfectly still in a pose that you hold for a very long time. Asanas, by contrast, are alive, in each moment a unique expression of the human being doing them.
Opening one’s self to a feeling of inner peace amid the relative intensity of the asana practice— being calm and soft while strong and stable— takes the practice to a deeper level. The breath itself starts to become a mantra in the movement meditation that is asana practice. In this way the practice is one of meditative awareness in which one is more fully and consciously attuned to what is happening in the moment.
This experiential process— not the religious worship of a deity or insistence on precise form in held poses— is what makes asana practice itself a transformational or spiritual practice. And it is precisely here, in creating a space that encourages the cultivation of clearer awareness, that the yoga teacher becomes an awareness facilitator.
Source: Yoga Sequencing: Designing Transformative Yoga Classes by Stephens, Mark (2012-09-18).