Yoga Sutras of Patanjali translated by Yogi Kalinath - Chapter 2, Verse 53
Pada 2, Sutra 53
Dhaaranaasu ca yogyataa manasah
And the mind becomes fit for concentration.
When the Inner Light of Consciousness is revealed then the mind, being an extension of Consciousness itself, becomes fit for concentration as it is no longer disturbed by its own vagaries.
The mind is normally dispersed either externally through its interactions with the senses or internally by its sense of insecurity and emotional connection to the past and future. Control of the life-force (Prana which animates our psychosomatic self) by regulating our breaths pacify the whirlpools of the mind allowing the mind to become one-pointed or concentrated. A tamed mind is truly an effective mind.