अपाने जुह्वति प्राण प्राणेऽपानं तथाऽपरे। प्राणापानगती रुद्ध्वा प्राणायामपरायणाः ॥

apāne juhvati prāṇaṁ prāṇe ’pānaṁ tathāpare prāṇāpāna-gatī ruddhvā prāṇāyāma-parāyaṇāḥ apare niyatāhārāḥ prāṇān prāṇeṣhu juhvati sarve ’pyete yajña-vido yajña-kṣhapita-kalmaṣhāḥ

Yet others pour as sacrifice the outgoing breath in the incoming, and the incoming in the outgoing, restraining the flow of the outgoing and incoming breaths, solely absorbed in the control of breathing.

In plain words: Krishna says some people make their offering through breath control. They carefully guide and pause the breath going in and out, and give their full attention to it.