Bhashya
तथैव उपासनान्तरं सत्यस्य ब्रह्मणो विशिष्टफलमारभ्यते— विद्युद्ब्रह्मेत्याहुः ।विद्युतो ब्रह्मणो निर्वचनमुच्यते— विदानात् अवखण्डनात् तमसो मेघान्धकारं विदार्य हि अवभासते, अतो विद्युत् ; एवंगुणं विद्युत् ब्रह्मेति यो वेद, असौ विद्यति अवखण्डयति विनाशयति पाप्मनः, एनमात्मानं प्रति प्रतिकूलभूताः पाप्मानो ये तान् सर्वान् पाप्मनः अवखण्डयतीत्यर्थः ।य एवं वेद विद्युद्ब्रह्मेति तस्यानुरूपं फलम् , विद्युत् हि यस्मात् ब्रह्म ॥
In plain words
In just the same way, one more meditation begins here, on this same Brahman as Truth, and it carries its own particular reward — 'Lightning is Brahman,' they say. Here is why it is called lightning: it tears the cloud's darkness open and shines straight through — that splitting-open is exactly what the word means. Whoever knows Brahman under this quality, as lightning, cuts through, splits apart, and destroys every sin standing against him, all of them, none left over. Know 'Lightning is Brahman' this way, and the fruit you receive matches exactly what you have come to know — because lightning, since that is simply what it is, truly is Brahman.
Close to the Sanskrit
In the same way, another meditation, on Brahman as Truth, is begun here, [yielding] a distinctive fruit — 'Lightning is Brahman,' they say. The etymology of lightning as Brahman is now stated: from splitting, from cutting through, having split apart the darkness of the cloud, it indeed shines forth; hence 'lightning.' He who knows Brahman possessed of this quality, as lightning — he cuts, splits apart, destroys the sins, standing opposed to this self, all of them; that is the meaning. He who thus knows 'Lightning is Brahman' [receives] a fruit corresponding to that knowledge — for lightning, indeed, since that is what it is, is Brahman.