Yogic dhyana commences with pranayama. Pranayama to samadhi is a single continuous process.
Pranayama regulates breath by desha (region), kala (duration), samkhya (cycles).
Breath and mind: two sides of the same coin. Breath assists comprehension.
Pranayama uses vehicle of the breath. Breath & mind must benefit from asana.
Asana + shvasayama (breath restraint)—prerequisite for pranayama & dhyana.