Bhagwad Geeta sings:
स्पर्शान्कृत्वा बहिर्बाह्यांश्चक्षुश्चैवान्तरे भ्रुवोः ।
प्राणापानौ समौ कृत्वा नासाभ्यन्तरचारिणौ ॥
विगतेच्छाभयक्रोधो यः सदा मुक्त एव सः ॥
sparśān kṛtvā bahir bāhyāmś
cakṣuś cai’vā’ntare bhruvoḥ,
prāṇāpānau samau kṛtvā
yaḥ sadā mukta eva saḥ
“That sage is liberated for ever
who shuts out of his mind all objects of sensual pleasure, keeps his eyes centered between the two brows,
regulates his pran and apan,
conquers his senses, mind and intellect,
whose mind is fixed on salvation.”
Sri Krishn reminds Arjun of the vital need of excluding from the mind all thoughts of external objects as well as of keeping eyes fixed steadily between the two brows. Keeping eyes between the brows does not simply mean concentrating them at something.
It is rather that while the worshiper is sitting erect,his eyes should be pointed ahead in a straight line from the midpoint between the brows; they should not wander about restlessly and look right and left. Keeping the eyes aligned with the ridge of the nose- we must be careful that we do not start watching the nose-and balancing pran against apan and keeping the eyes steadily fixed all the while, we should direct the vision of mind, the Soul, to the breath and let him watch it:
when does the breath go in,
how long is it held-if it is held in for only half a second,
we should not try to prolong it by force,
and how long does it stay out?
It is hardly necessary to say
the name in the breath will ring audibly.
Thus when the vision of mind learns to concentrate steadily
on the inhaled and exhaled breath,
breathing will gradually become constant,
There will be then neither generation of inner desires
assaults on the mind and heart
by desires from external sources.
Thoughts of external pleasure have already been shut out; now there will not even arise inner desires.
Contemplation then stands steady and straight
like a stream of oil.
A stream of oil does not descend like water, drop by drop;
it comes down in a constant, unbroken line.
Similar to this is the motion of the breath
a sage of attainment.
So the man, who has balanced his pran and apan, conquered his senses, mind and intellect, freed himself from desire, and fear and anger, perfected contemplative discipline, and taken refuge in salvation,
[Revered Swami Adgadanandji Paramhans]