This question has been raised to me
with the request
to throw light upon this.
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Lord Krishn has taught the necessity of contemplating
the incoming and outgoing breath
for the realization of this sage’s state in Bhgavad Gita.

In Chapter 4, he has taught us of offering pran to apan,
of sacrificing apan to pran, and of the regulation
of
both the life winds while giving the account of the process of yagya.
The same subject is taken up here.

He has taught in Bhagavad Gita :

“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,
and
whose mind is fixed on salvation.”

Lord 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 worshipper 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
(Inhale and Exhail breath)
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 that the intoning name
consisting of two or two and half letters
(preferably Om or Ram)
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, firm, and balanced.
There will be then neither generation of inner desires
nor 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 of 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, is ever-liberated.

This is in brief regulation of the incoming and outgoing breath.

Bow down in lotus feet of most revered Gurudev for such teaching to me.