Pranayama to control the mind
The object of Yoga is the control of the mind and its avocations "Chitta Vrittis". In order to control the mind, we must catch hold of it with the help of "Prana".
As we tie a playful calf with the help of a rope, we use Pranayama to tie or control the mind. Similarly Pranayama is possible by the controlled regulation of inhalation, exhalation force, and the mind.
To sum up, the mind is controlled by 'Prana' and the Prana is controlled by the breath. Thus, the control of breath ultimately leads to the control of the mind. Pranayama is a prerequisite for Samadhi the highest step in Yoga.