Bhashya
क्षीरोदार्णवशायिनं ना चासौ केसरी च नृकेसरी तंनृकेसरिणम् ।नृकेसरिमिति छान्दसम् ।योगिवदासीनं शेष-भोगमस्तकपरिवृतं रहस्यकल्पान्तरपर्यालोचनया अस्मिन्नेवस्थाने शाखान्तरश्रुत्यन्तरपर्यालोचनया आसीनं शेषभोगमू-र्धावृतम् ।द्विशाखगामिनी इयमुपनिषत् भृगुशाखायाम्आङ्गिरसशाखायां च ।परमं पदं परमाश्रयभूतम् ।जानीयादिति सर्वत्रानुषज्यते ।सामेत्यादि गच्छतीय-न्तम् उक्तार्थं भवति ।यत्साम जानीयात् तदुक्तगुण-विशिष्टे नृकेसरिणि साङ्गं न्यस्तं जानीयादित्यर्थः ।इदा-योर्विकल्पः ।यद्वा आचार्यशब्दो वेदमभिधत्ते, 'तस्मादाचा-र्यवच्चः प्रमाणम्' इत्यादौ प्रयोगात् ; आचार्यमुखेन वेदमुखेनसामोद्धारवाक्यकदम्बकेनेत्यर्थः ।स तेनैव स्वयमेव सामप-रिज्ञानेन संसारान्मुच्यते, मोचयति अन्यं वा साम ज्ञापयन् ।सामपरिज्ञानात् सरागोऽपि मुमुक्षुर्भवति जपात्, संख्यानुपा-दानात् सकृदेव सामजपात् ।यत्साङ्गेन साम्ना पारमेश्वरंशरीरं क्षीरोदार्णवशायित्यादिपदकदम्बकव्याकृतम्, तेनैवशरीरेण देवतादर्शनं करोति देवता प्रत्यक्षा भवति ।तस्मादिद-मेव साङ्गं सामैव मुख्यं द्वारं देवतादर्शने तद्व्याकृतौ च ; कलौपापबाहुल्येन अन्येषां सामरहितानां केवलमूलमन्त्रपरिज्ञा-तॄणां देवतादर्शनं झटिति न भवतीत्यर्थः ।यस्मादिदं साङ्गंसाम देवतादर्शने देवताकारव्याकृतौ च मुख्यमेव द्वारम्,तस्मादिदं साङ्गं साम जानीयात् ; मुमुक्षुर्भवति मानुषानन्दा-रूढोऽपि एतत्परिज्ञानात् मानुषानन्दं विहाय मोक्षेप्सुर्भ-वति ।यद्वा मुमुक्षुः एतज्जानीयात् इत्यन्वयः ।तस्य भव-तीति फलनिर्देशः ।भू सत्तायामिति धातोः रूपम् ।साका-रब्रह्मोपासनाद्वारं सन्मात्रतां प्राप्नोतीत्यर्थः ॥
In plain words
Let's take the words apart. A man, and a lion -- put them together and you get Nṛkesarī, the Man-Lion; that is who is meant. The shorter form you see in the verse is simply how the Vedic language sometimes bends the rule. 'Seated like a yogin' -- one old tradition pictures him ringed by the crest of Śeṣa's coiled hood; another recension, another scripture, pictures him seated in that same spot, covered by that same serpent's hood. This Upaniṣad itself comes down to us along two lines, the Bhṛgu line and the Āṅgirasa line. 'The supreme station' simply means the ultimate ground everything rests on. And the phrase 'one should know' is meant to be carried silently through every line that follows -- everything from 'sāma' to 'attains' means exactly what we've already said: know this chant as deposited, whole, in Nṛkesarin, under all his stated qualities. (Either vowel-ending of his name is fine, by the way.) Now, there's another way to take 'through the mouth of the teacher': the word 'teacher' can itself mean the Veda -- there's precedent for that -- so it would mean 'through the words of the Veda that draw out this chant.' Either way, simply by knowing the chant himself, a person is freed from the wheel of rebirth -- or frees someone else, by teaching it to them. And this works even for someone who still carries desire: just from knowing the chant, through reciting it -- and no particular number of times is required, even once is enough -- that person starts wanting freedom. Because this chant, limbs and all, lays out the very body of the Supreme Lord, in that same body the practitioner comes face to face with the deity itself -- the deity becomes something seen, not just believed in. That is why this chant is the one true door to that vision, and to what it reveals: in an age as clouded by wrongdoing as this one, people who only know the bare root-mantra, without this chant, don't get there quickly. Because this is the one true door -- to seeing the deity and to seeing its very form -- know this chant whole, and you become someone who genuinely wants liberation. Even someone who has climbed all the way up to human happiness will, once they know this, set that happiness down and start wanting freedom instead. Or, read differently: it is the one who already wants liberation who should learn this. Either way, the phrase 'it belongs to him' is simply naming the reward -- 'bhavati' here is just the ordinary verb 'to be,' nothing more technical. What it comes down to is this: through meditating on Brahman as embodied form, a person arrives, in the end, at Being itself, with nothing added.
Close to the Sanskrit
'Kṣīrodārṇavaśāyinam': he who is a man (nara) and he who is a lion (kesarī) -- together, Nṛkesarī; him, [in the accusative,] Nṛkesariṇam. 'Nṛkesarim' [without the long vowel] is a Vedic irregularity. '[He who is] seated like a yogin' means: encircled by the crest of the coils of the couch that is Śeṣa -- by the reflection of one esoteric tradition; or, 'seated in this very place' -- by the reflection of a different recension and a different scriptural text -- means seated, covered over by the hood of the couch that is Śeṣa. This Upaniṣad proceeds along two branches: in the Bhṛgu recension and in the Āṅgirasa recension. 'The supreme station' means the supreme substratum. '[One] should know' is to be carried forward everywhere. [The stretch] beginning with 'sāma' and ending with 'gacchati' has the meaning already stated. 'The sāman which one should know' means: one should know it as deposited, together with its limb, in Nṛkesarin, who is qualified by the aforesaid attributes -- this is the meaning. There is a free option between the vowels 'i' and 'ī' [in this name]. Or else, the word 'ācārya' denotes the Veda, on the strength of usage such as 'therefore the word of the teacher is authoritative'; so 'through the mouth of the teacher' means 'through the mouth of the Veda,' that is, through the collection of sentences that extract the sāman. He, by that very means, by his own knowledge alone of the sāman, is released from transmigration, or he releases another by making him know the sāman. From the knowledge of the sāman, even one who still has attachment becomes desirous of liberation, through recitation -- since no number [of repetitions] is prescribed, even from a single recitation of the sāman. Since, by the sāman together with its limb, the body of the Supreme Lord, set forth by the collection of terms beginning with 'kṣīrodārṇavaśāyī,' [is disclosed] -- by that very body he brings about vision of the deity; the deity becomes directly perceived. Therefore this sāman together with its limb alone is the principal gateway to the vision of the deity and to its disclosure; in the Kali age, because of the abundance of sin, for others who are without the sāman, who know only the root mantra, the vision of the deity does not come about swiftly -- this is the meaning. Since this sāman together with its limb is the principal gateway to the vision of the deity and to the disclosure of the deity's form, therefore one should know this sāman together with its limb; he becomes one desiring liberation -- even one mounted upon human happiness, from this very knowledge, having abandoned human happiness, becomes one desiring liberation. Or else, the construal is: 'one who is [already] desirous of liberation should know this'; 'it belongs to him' states the result. This is a form of the root 'bhū,' [used] in the sense of existence. The meaning is: through the gateway of meditation upon Brahman-with-form, he attains the state of mere Being.