In plain words
For one whose attachment has gone, who is free and whose mind rests in jñāna, action performed for yajña dissolves completely.
Word for word
For one whose attachment is gone, who is free, whose mind is established in knowledge, who acts for yajña, action dissolves entirely.
The commentators
Bhagavadgītābhāṣyam
‘त्यक्त्वा कर्मफलासङ्गम्’(भ. गी. ४ । २०) इत्यनेन श्लोकेन यः प्रारब्धकर्मा सन् यदा निष्क्रियब्रह्मात्मदर्शनसम्पन्नः स्यात् तदा तस्य आत्मनः कर्तृकर्मप्रयोजनाभावदर्शिनः कर्मपरित्यागे प्राप्ते कुतश्चिन्निमित्तात् तदसम्भवे सति पूर्ववत् तस्मिन् कर्मणि अभिप्रवृत्तस्य अपि ‘नैव किञ्चित् करोति सः’(भ. गी. ४ । २०) इति कर्माभावः प्रदर्शितः । यस्य एवं कर्माभावो दर्शितः तस्यैव —
गतसङ्गस्य सर्वतोनिवृत्तासक्तेः, मुक्तस्य निवृत्तधर्माधर्मादिबन्धनस्य, ज्ञानावस्थितचेतसः ज्ञाने एव अवस्थितं चेतः यस्य सोऽयं ज्ञानावस्थितचेताः तस्य, यज्ञाय यज्ञनिर्वृत्त्यर्थम् आचरतः निर्वर्तयतः कर्म समग्रं सह अग्रेण फलेन वर्तते इति समग्रं कर्म तत् समग्रं प्रविलीयते विनश्यति इत्यर्थः ॥ २३ ॥
In plain words
This verse, 'having abandoned attachment to the fruit of action,' concerns whoever, with action already under way, has the vision of the actionless Brahman-Self arise in him. For such a person, seeing no agent, no action, and no purpose left anywhere, giving up action would be the natural consequence — but if for some reason that isn't possible, and he goes on acting as before, 'he does nothing at all,' as the text says — meaning the absence of action has been shown to belong to him. And it is precisely this person, in whom the absence of action has been shown in this way—
For someone whose attachment has vanished on every side, who is free, whose bondage to merit and demerit has ceased, whose mind rests fixed in knowledge alone — for such a person, when he acts for the sake of sacrifice, performing it and carrying it out, that action — in its entirety, fruit and all — simply dissolves, perishes. That is the meaning.
Close to the Sanskrit
By this verse, 'having abandoned attachment to the fruit of action' (Bhagavadgītā 4.20) — for whoever, having action already under way, should the vision of the actionless Brahman-Self arise in him, then for him, the Self, who sees the absence of any agent, action, or purpose, the abandonment of action being the consequence; for some reason, that being impossible, though he remains engaged in that action as before, 'he does not do anything at all' (Bhagavadgītā 4.20) — the absence of action has been shown for him. And whoever has thus had the absence of action shown to him, of that very person —
Of one whose attachment has gone — whose attachment has ceased on every side — of the liberated one, freed from the bondage of merit, demerit, and the rest — of one whose mind is fixed in knowledge, that is, he whose mind alone is fixed in knowledge is this one of mind fixed in knowledge — of him, performing, carrying out, action for the sake of sacrifice, for the accomplishing of the sacrifice: 'action exists together with its fruit' — that entire action, that whole action, dissolves, perishes. This is the meaning.