Bhagavad Gita · Dhyana Yoga (Atma-samyama Yoga)Chapter 6, Shloka 13 (6.13)

Shankaracharya — the dropped "iva" (as if). Samaṃ dhārayataḥ calanaṃ saṃbhavati ataḥ viśinaṣṭi acalam iti — even while holding it straight, movement is possible, so "unmoving" is also said. Important: svaṃ nāsikāgraṃ saṃprekṣya samyak prekṣaṇaṃ darśanaṃ kṛtveva iti, iva-śabdo lupto draṣṭavyaḥ — "as if looking" — the word "iva" (as if) should be understood as hidden here. Na hi sva-nāsikāgra-saṃprekṣaṇam iha vidhitsitam, kiṃ tarhi cakṣuṣo dṛṣṭi-saṃnipātaḥ — staring at the tip of the nose is not the instruction; the meaning is that the eyes' gaze settles and stays steady. The argument: sva-nāsikāgra-saṃprekṣaṇam eva ced vivakṣitam manaḥ tatraiva samādhīyeta nātmani — if looking at the tip of the nose were the goal, the mind would settle on the nose, not on the self; and 6.25 says ātma-saṃsthaṃ manaḥ kṛtvā.

Acharya Ramanuja (6.13–6.14 combined). Kāya-śiro-grīvaṃ samam acalaṃ sāpāśrayatayā sthiraṃ dhārayan — trunk, head and neck straight, unmoving, steady with a support; diśaś ca anavalokayan svaṃ nāsikāgraṃ saṃprekṣya.

The difference: Shankara's "iva" explanation is very important in practice — the gaze is not to be forced onto the nose; Ramanuja again stresses sāpāśraya (support).