B.G 4.29

अपाने जुह्वति प्राणं प्राणेऽपानं तथापरे। प्राणापानगती रुद्ध्वा प्राणायामपरायणाः ॥२९॥

apāne juhvati prāṇaṁ prāṇe'pānaṁ tathāpare। prāṇāpānagatī ruddhvā prāṇāyāmaparāyaṇāḥ ॥29॥

[अपाने जुह्वति प्राणं प्राणे अपानं तथा परे। प्राण अपान गती रुद्ध्वा प्राण आयाम परायणाः॥
apāne = in the exhalation; juhvati = offers; prāṇaṁ = inhalation; prāṇe = in inhalation; apānaṁ = the exhalation; tathā = in the same way; pare = in others; prāṇa = inhalation; apāna = exhalation; gatī = pace; ruddhvā = having controlled; prāṇa-āyāma = called 'prāṇa-āyāma', the breathing exercise; parāyaṇāḥ = devoted to;]

Offers exhalation into inhalation, and inhalation into exhalation. In this way, having controlled the pace of inhalation and exhalation, they become dedicated to the exercise of breath, called 'prāṇa-āyāma'.

Gīta Bhāshya 4.29

Breathing exercise mentioned is 'prāṇa-āyāma' that involves practice of controlled breathing with retention of breath, called 'Kumbhaka'.

अपरे प्राणायामपरायणाः प्राणम् अपाने जुह्वति, अपानं च प्राणे। कुम्भकस्था एव भवन्तीत्यर्थः ॥२९॥

apare prāṇāyāmaparāyaṇāḥ prāṇam apāne juhvati, apānaṁ ca prāṇe। kumbhakasthā eva bhavantītyarthaḥ ॥29॥

[apare = others; prāṇāyāmaparāyaṇāḥ = who are devoted to 'prāṇa-āyāma'; prāṇam = inhalation into; apāne = exhalation; juhvati = offer; apānaṁ = exhalation into; ca = and; prāṇe = inhalation'। kumbhakasthā = established in retention of breath; eva = only; bhavantītyarthaḥ = the become, is the meaning;]

Others who are devoted to 'prāṇa-āyāma' practice offer inhalation into exhalation and exhalation into inhalation. They become established in retention of breath, i.e. 'Kumbhaka' state, is the meaning.