During iṣṭiyāga, while giving ghee as āhuti, the kind of mantra which is uttered both in high and low volume, is called upāṃśu yāga. Professor Amarkumar Chattopadhyay has talked about the significance of Upāṃśu-yāga–
upāṃśu is ̍karaṇavad aśavdam amanaḥprayogam

To utter words, one has to move the tongue, lips, and the mouth, but the words uttered will be so faintly spoken, that nobody expect the person who is uttering, will be able to hear them. But it is not silent, it is also said in another of its characteristics–
śanair uccārayen mantraṃ mandram oṣṭhau pracalayet.
aparair aśruta kiñcit sa upāṃśu-japaḥ smṛtaḥ.

In the sūtra, the mantra which is spoken of, is based on karma to be performed, as denoted by the dhātu(s) like ̍jap̍. anu-mantra (abhi-mantra), ā- pyā, upa-sthā and so on. These have other characteristics as well–

japam uccāraṇṃ vidyāt kratvarthaṃ api tad bhavet
arthataḥ kāryalābhaśced artha eva kratorbhavet
mantaṃ uccārayanneva mantrāthatvena saṃsmaret
śoṣiṇaṃ tanmanā bhūtvā syād etad anumantraṇam.
etad evābhimantrasya lakṣaṇañcekṣaṇādhikam
adbhiḥ saṃsparśanādhikyāt tad evāpyāyanaṃ smṛtam
upasthānaṃ tad eva syat prṇatisthānasaṃyutam
vāhyaṃ kāryaṃ yad eteṣu mantrakāle kriyate tat.

For the purpose of yajña, one kind of mantra is uttered in upāṃśu-svara, it is called japa. If the desired act is fulfilled through this meaning of the japamantra, the yajña also becomes meaningful. Meditatively contemplating on the god who is the subject of the mantra, while uttering it, is called anumantraṇa. In case of abhimantra, the Deity is to be remembered with full attention, and at the same time, attention is to be given to the act which is being performed. If, keeping eye to the related object one sprinkles water on that, and at the same time, contemplating the Deity, is called āpyāyana. Upasthāna is offering ̍praṇāma with folded hands. These acts of contemplation, looking at the object, sprinkling water are to be performed while uttering the mantra. Anumantraṇa, Āpyāyana, Upasthāna– these are Karmakaraṇa (Karmasampṛkta) or karma-related mantra. But since these mantras are mentioned separately, it can be understood that in case of these karmakarṇa mantra, the related karma is to be performed while continuing the utterance of the mantra, unlike other karmakarṇa mantra(s).