In plain words
When one sees the diverse existence of beings as rooted in one and their expansion from that alone, one becomes Brahman.
Word for word
When he sees the separate existence of beings as resting in the One, and the spreading forth from that alone, then he becomes Brahman.
यदा भूतपृथग्भावमेकस्थमनुपश्यतितत एव च विस्तारं ब्रह्म सम्पद्यते तदा
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
पुनरपि तदेव सम्यग्दर्शनं शब्दान्तरेण प्रपञ्चयति —
यदा यस्मिन् काले भूतपृथग्भावं भूतानां पृथग्भावं पृथक्त्वम् एकस्मिन् आत्मनि स्थितं एकस्थम् अनुपश्यति शास्त्राचार्योपदेशम् , अनु आत्मानं प्रत्यक्षत्वेन पश्यति ‘आत्मैव इदं सर्वम्’(छा. उ. ७ । २५ । २) इति, तत एव च तस्मादेव च विस्तारं उत्पत्तिं विकासम् ‘आत्मतः प्राण आत्मत आशा आत्मतः स्मर आत्मत आकाश आत्मतस्तेज आत्मत आप आत्मत आविर्भावतिरोभावावात्मतोऽन्नम्’(छा. उ. ७ । २६ । १) इत्येवमादिप्रकारैः विस्तारं यदा पश्यति, ब्रह्म सम्पद्यते ब्रह्मैव भवति तदा तस्मिन् काले इत्यर्थः ॥ ३० ॥
In plain words
And now, once again, the same correct seeing gets unfolded a little further — this time in different words.
Whenever a person, guided by scripture and by a teacher, actually sees that every apparent difference between beings — all that separateness — really rests in one single Self; and when he sees that Self directly, immediately, in line with the teaching, 'this Self alone is everything there is'; and when he further sees that everything unfolds, arises, blossoms out from that very Self — life-breath from the Self, hope from the Self, memory from the Self, space from the Self, fire from the Self, water from the Self, the whole coming-and-going of appearance and disappearance from the Self, even food from the Self — when he sees all of this unfolding this way, in every one of these instances and more like them — at that moment, exactly then, he becomes Brahman. He simply is Brahman. That's what this teaching means.
Close to the Sanskrit
Once more, he elaborates that very same correct vision, in different words —
When, at whichever time, he sees, following the instruction of scripture and teacher, the separateness of beings, the separateness, the individuality of beings, as resting, situated, in the one Self; when he sees the Self directly, following that same instruction — 'the Self alone is all this' (Chāndogya Up. 7.25.2) — thus; and from that very Self, from that very source, the expansion, the origination, the unfolding — 'from the Self, life-breath; from the Self, hope; from the Self, memory; from the Self, space; from the Self, fire; from the Self, water; from the Self, appearance and disappearance; from the Self, food' (Chāndogya Up. 7.26.1) — by such means and so on, when he sees this expansion — he attains the state of Brahman, becomes Brahman itself: then, at that time, that is the meaning.