Others offer prana into apana, and apana into prana. Stopping the movement of both, they devote themselves to breath control.
Explanation
Krishna is continuing his list of different kinds of sacrifice (yajña), meaning disciplined practices offered as worship. Here he describes breath-control practitioners. They offer the outgoing breath (prāṇa) into the incoming breath (apāna) and the reverse, then hold both still in the discipline of prāṇāyāma, or breath regulation. Traditions differ on how to read the terms prāṇa and apāna and the order of the breaths, but most take the verse as describing a real yogic breathing practice.