II.52 Pranayama removes the veil covering the light of knowledge and heralds the dawn of wisdom.
II.53 The mind then becomes fit for concentration (dharana)
dharanasu ca yogyata manasah
III.1 Fixing the consciousness on one point or region is concentration.
desa bandhah cittasaya dharana
Dharana means focus of attention. Focusing the attention on a chosen point or area, within or outside the body, is concentration. “By it the functions of the mind are controlled and brought to one focal point.” – Iyengar.
Focus on the breath and action in asana is a form of dharana. Doing the shapes of the poses without attention, focus and diligence in action is not yoga. My teacher has said that yoga is not stretching, it is working with intelligence, or awareness. Bringing intelligence to every cell in the body. Yes, even the hamstrings!