Gita 04.29 explained
apāne juhvati prāṇaṁ
prāṇe ’pānaṁ tathāpare
prāṇān prāṇeṣu juhvati
apāne — in the air which acts downward; juhvati — offer; prāṇam — the air which acts outward; prāṇe — in the air going outward; apānam — the air going downward; tathā — as also; apare — others; prāṇa — of the air going outward; apāna — and the air going downward; gatī — the movement; ruddhvā — checking; prāṇa–āyāma — trance induced by stopping all breathing; parāyaṇāḥ — so inclined; apare — others; niyata — having controlled; āhārāḥ — eating; prāṇān — the outgoing air; prāṇeṣu — in the outgoing air; juhvati — sacriﬁce.
Still others, who are inclined to the process of breath restraint to remain in trance, practice by offering the movement of the outgoing breath into the incoming, and the incoming breath into the outgoing, and thus at last remain in trance, stopping all breathing. Others, curtailing the eating process, offer the outgoing breath into itself as a sacriﬁce.
Now Shri Krishna will talk about controlling the breath
Based on continuity Srila Prabhupad has cut the verses so here in BG 4.29 two lines are added and in 4.30 only two lines are there so in 4.29 we have apart from regular parts of verses a,b,c,d added e & f.
apāne juhvati prāṇaṁ -> Apane is the air that goes downwards
Prana -> is the air that goes outwards
So the idea is one offer outward air in inward and inward air in outward
prāṇe ’pānaṁ tathāpare
in this way one checks the movement of the air. One brings the movement of the air in the steady state of neutrality
Such a person is dedicated to Pranayam so this is one kind of Yoga
and others apare niyatāhārāḥ – they regulate their eating a lot and prāṇān prāṇeṣu juhvati and they offer their Prana in the air. They offer the outgoing air into itself.
So one may wonder about what is this offer into air, outgoing air etc. Srila Prabhupad has explained this briefly:
The apāna air goes downward, and the prāṇa air goes up. (Up means it comes up and goes out of the body), the prāṇāyāma-yogī practices breathing the opposite way until the currents are neutralized into pūraka, equilibrium. There is a particular way body breath and if one breathes in opposite way it will increase the will power and one will be able to regulate his body.
Offering the exhaled breath into the inhaled breath is called recaka. When both air currents are completely stopped, one is said to be in kumbhaka-yoga
These are all technicality which is not part of the yuga dharmas so we will not discuss these in details but this was one of the ritual or path used for gradual spiritual advancement. This is ultimately meant to attain Brahman.
In all these verses Srila Prabhupad maintains the system that we should practice Krishna Consciousness. Yes this are all valid rituals or process and they are part of vedic culture but the important point is, in this Kalyuga we practice Krishna Consciousness, the ultimate recommendation by Shri Krishna in Bhagwad Gita. When we look at all these process which Krishna gives or talks about we understand one part of the rational of the statement that Shri Krishna gives at the end of BG in 18.66 when he says Serva Dharmam Parityaja Mam Ekam Shranam Vraja..so why he says serva dharmam Parityaja because these are all the verities of religion present in vedic religion so Krishna says give them up just mam ekam sharnam vraja so what Shri Krishna gives at the end of Bhagavad Gita Srila Prabhupad gives throughout Bhagavad Gita, so Srila Prabhupad is not cancelling what Krishna is giving but Srila Prabhupad is giving the conclusion throughout the book. So we don’t get any misunderstanding we don’t lose out.
So Srila Prabhupad writes in Purport: Reducing the eating process is very helpful in the matter of sense control. And without sense control there is no possibility of getting out of the material entanglement.
This doesn’t mean that we have to intentionally starve ourselves, idea is not to eat so less that we don’t have any energy to do service for Krishna and cannot perform our responsibilities and to lose the opportunities that Shri Krishna sends our way, the important point is we don’t make eating a primary activity (always eating) or primary fantasy (when is next food coming), eating is one part and we don’t eat in large quantity. So Srila Prabhupad said by eating Prasadam we regulate senses and in prasadam also we keep quantity and quality under control that will help in keeping senses under control. So sense control is important or foundation for liberation