When breath is unsteady, the mind is unsteady. When breath is steady, the mind is steady. The yogi should therefore restrain the breath.
Breath is the bridge which connects life to consciousness, which unites your body to your thoughts. Whenever your mind becomes scattered, use your breath as the means to take hold of your mind again.
Fixity of attention depends on slow breathing; quick or uneven breaths are an inevitable accompaniment of harmful emotional states — fear, lust, anger.