Now try to understand Chapter overview by trying to understand what is happening:

In Arjun’s mind there were two options action or renunciation, So Sri Krishna has told him about how there can be action in renunciation these two need not be opposite it can be integrated, it need not be fragmented, When Sri Krishna speaks how it can be integrated then the question comes up is, is it that action alone will ultimately leads to liberation, so Karma Yoga alone can lead to liberation, it can, when a person is practicing Karma Yoga eventually the person can renounce the world and go to forest, when the person is a grihasta he lives in the world and acts in Karma Yoga. But later on when the person takes Vanaprastha and Sanyas the person may not be acting in Karma Yoga. That time the person may take up Gyana Yoga, Astanga Yoga the person may take up full time Bhakti Yoga also that can vary, the way Sri Krishna has explained Karmanev He sinsidhi, by Karma alone one can take up liberation that means one can stay in liberated stage even while practicing one’s work but eventually as Varnaashram progression is there one will renounce the world, so when one renounces the world what one does at that time to complete the process towards liberation, Sri Krishna is telling here instead of Gyana Yoga Sri Krishna has already told Dhukam Aptu Ayogta, Sri Krishna said if one is not practicing Karma Yoga one will get only Dhuka so here Sri Krishna is saying if one wants to practice liberation one should practice Astanga Yoga, this is described in two verses and it is elaborated in sixth chapter, there he will talk about how to renounce world through astanga yoga, in last verse of this chapter Sri Krishna will talk about Bhakti Yoga, this Bhakti Yoga will be elaborated in next section of Bhagvat Gita from Chapter 07 to Chapter 12.

So this is the progression in first 5 chapters Sri Krishna has established for Arjuna, action is better than renunciation, Karma Yoga is better than Gyana Yoga and when he does Karma Yoga and eventually he wanted to renounce the world that time he can practice Astanga Yoga or he can Practice Bhakti Yoga and Bhakti Yoga can be continuously practiced he need not have to change the process, that he will talk elaborately in later chapters 07 to 12. So now let us talk about a brief glimpse of Astangs Yoga that Sri Krishna gives over here.

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Bg 5.27-28

sparśān kṛtvā bahir bāhyāṁś

cakṣuś caivāntare bhruvoḥ

prāṇāpānau samau kṛtvā

nāsābhyantara-cāriṇau

yatendriya-mano-buddhir

munir mokṣa-parāyaṇaḥ

vigatecchā-bhaya-krodho

yaḥ sadā mukta eva saḥ

Word for word: 

sparśān — sense objects, such as sound; kṛtvā — keeping; bahiḥ — external; bāhyān — unnecessary; cakṣuḥ — eyes; ca — also; eva — certainly; antare — between; bhruvoḥ — the eyebrows; prāṇa–apānau — up- and down-moving air; samau — in suspension; kṛtvā — keeping; nāsa–abhyantara — within the nostrils; cāriṇau — blowing; yata — controlled; indriya — senses; manaḥ — mind; buddhiḥ — intelligence; muniḥ — the transcendentalist; mokṣa — for liberation; parāyaṇaḥ — being so destined; vigata — having discarded; icchā — wishes; bhaya — fear; krodhaḥ — anger; yaḥ — one who; sadā — always; muktaḥ — liberated; eva — certainly; saḥ — he is.

Translation: 

Shutting out all external sense objects, keeping the eyes and vision concentrated between the two eyebrows, suspending the inward and outward breaths within the nostrils, and thus controlling the mind, senses and intelligence, the transcendentalist aiming at liberation becomes free from desire, fear and anger. One who is always in this state is certainly liberated.

 

Now first Sri Krishna said that bahir bāhyāṁś, keep outside, what to keep outside? sparśān kṛtvā, sparśān represents objects such as sound, so sparśān kṛtvā normally we may say sparśā refers to touch but sparsa in general refers to contact of senses to sense objects, Sri Krishna has used Sparsha in general sense in 5.22, in same sense Sri Krishna has used Sparsha, sparśān kṛtvā bahir bāhyāṁś, keep external sense object external, don’t focus on them,

cakṣuś caivāntare bhruvoḥ, so what one should do with eyes, āntare bhruvoḥ, fix the eyes in the middle of the two eyebrows, Sri Krishna is talking about Yogashana where one does pratyahar, cutting out the sense objects, then

prāṇāpānau samau kṛtvā, regulate the breath and nāsābhyantara-cāriṇau, this verse 27 talks about the physical control one should do, at physical level what one should do to regulate the breath turn away the sense object, fix the eyes in between the two eyebrows. In the next line He will talk about what we should do internally. 

yatendriya-mano-buddhir, bring the mind, senses and intelligence under control yata, munir mokṣa-parāyaṇaḥ, the person should be mokṣa-parāyaṇaḥ understand that nothing in this material world makes me happy, I have to go beyond mokṣa-parāyaṇaḥ, vigatecchā-bhaya-krodho, such a person becomes free from icchā, Bhaya and Krodha, now Sri Krishna has talked about icchā and Krodha earlier, so where is Bhaya coming in now, icchā can be equated with Kama, so Kama Krodha Vimuktanam Sri Krishna has talked in earlier verse, so from where Bhaya is coming from, these three Sri Krishna talked about regularly, he has talked about Vita Raga Bhaya Krodha twice. Once in 4.10, and in 2.56. So these three are referred together as Raga, Bhaya & Krodha. Raga refers to attachment, O, I wanted to enjoy this world, Krodha refers to who will stop me in enjoying this world and Bhaya is I don’t know what is there in spiritual realm, I don’t want to explore this, so these are all obstacles, Vigat Eicha Bhaya Krodha, one should not have desire in this world, one should not be frustrated in this world, if somebody renounce because he is frustrated in this world that aversion will also keep one’s thought fix in this world, if someone wanted to renounce it should not be because of frustration of this world it should be because of attraction of another world. If it is only because of the frustration of this world then it cannot be sustained. To sustain it has to be based on attraction of another world, that is why one has to become free from Krodha also, vigatecchā-bhaya-krodho, bhaya is I don’t want to fall in the complexity of relationship so I don’t want to get into this business let me just out of the whole things, let me merge in void of something like that. Sri Krishna is not saying such a person will be liberated, he is saying such a person is already liberated, is always liberated, yaḥ sadā mukta eva saḥ, yaḥ sadā who is in this stage mukta eva saḥ. Once one goes to Pratayhara stage once shuts out the external senses objects and fixes mind inwards, then after that if one is free from anarthas then one is as good as liberated. Here Srila Prabhupada is also talking about keeping the eyes open and not closing the eyes as one may fall asleep. Srila Prabhupada has also written that Krishna Consciousness is better than astanga Yoga.