In plain words
Some offer the outgoing breath into the incoming breath, and others the incoming into the outgoing, restraining the movements of prāṇa and apāna and devoted to prāṇāyāma.
Word for word
Others likewise offer prāṇa in apāna, apāna in prāṇa, having checked the movements of prāṇa and apāna, intent on prāṇāyāma.
The commentators
Bhagavadgītābhāṣyam
किञ्च —
अपाने अपानवृत्तौ जुह्वति प्रक्षिपन्ति प्राणं प्राणवृत्तिम् , पूरकाख्यं प्राणायामं कुर्वन्तीत्यर्थः । प्राणे अपानं तथा अपरे जुह्वति, रेचकाख्यं च प्राणायामं कुर्वन्तीत्येतत् । प्राणापानगती मुखनासिकाभ्यां वायोः निर्गमनं प्राणस्य गतिः, तद्विपर्ययेण अधोगमनम् अपानस्य गतिः, ते प्राणापानगती एते रुद्ध्वा निरुध्य प्राणायामपरायणाः प्राणायामतत्पराः ; कुम्भकाख्यं प्राणायामं कुर्वन्तीत्यर्थः ॥ २९ ॥
In plain words
And there's more —
'Into apāna' — that is, into the function of apāna — they pour 'prāṇa,' the function of prāṇa; this simply means they practice the breath-control called inhalation. 'Into prāṇa, apāna' — likewise, others do it the other way around, meaning they practice the breath-control called exhalation. 'The two courses of prāṇa and apāna': the breath going out through mouth and nostrils is the course of prāṇa; the reverse of that, moving downward, is the course of apāna. Checking and restraining both of these courses, devoted entirely to breath-control — this means they practice the breath-control called retention. That is the meaning.
Close to the Sanskrit
Moreover —
'Into apāna' — into the function of apāna — they offer, they cast, 'prāṇa' — the function of prāṇa — meaning they perform the prāṇāyāma called inhalation. 'Into prāṇa, apāna' — likewise others offer this way — meaning they perform the prāṇāyāma called exhalation. 'The two courses of prāṇa and apāna': the outgoing of the breath through mouth and nostrils is the course of prāṇa; by the reverse of that, the downward movement is the course of apāna; those two courses of prāṇa and apāna, having checked, having restrained them, being devoted to prāṇāyāma, intent on prāṇāyāma — meaning they perform the prāṇāyāma called retention. This is the meaning.