In plain words
Constantly keeping the self collected in this way, the yogin, freed from impurity, easily experiences the limitless happiness of contact with Brahman.
Word for word
Thus always collecting the mind, the yogin, with stain gone, easily attains the limitless happiness of contact with Brahman.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
युञ्जन् एवं यथोक्तेन क्रमेण योगी योगान्तरायवर्जितः सदा सर्वदा आत्मानं विगतकल्मषः विगतपापः, सुखेन अनायासेन ब्रह्मसंस्पर्शं ब्रह्मणा परेण संस्पर्शो यस्य तत् ब्रह्मसंस्पर्शं सुखम् अत्यन्तम् अन्तमतीत्य वर्तत इत्यत्यन्तम् उत्कृष्टं निरतिशयम् अश्नुते व्याप्नोति ॥ २८ ॥
In plain words
Practising yoga in this way — meaning by the graded method already laid down — the yogin, with the obstacles to yoga cleared away, always, that is at every moment, holding the self; free of taint, that is free of sin; easily, that is without any strain — he attains the happiness that touches Brahman. Take that compound apart: the happiness whose contact is with Brahman, the supreme, is 'Brahman-touching' happiness. And atyanta: it abides beyond every limit, and that is why it is called atyanta — the highest, with nothing beyond it. He attains it; that is, he comes to pervade it.
Close to the Sanskrit
Practising yoga (yuñjan) thus (evam) — that is, by the sequence that has been stated — the yogin, freed from the obstacles to yoga, always (sadā), that is, at all times, [holding] the self, free of taint (vigatakalmaṣaḥ), that is, free of sin, easily (sukhena), that is, without exertion — the happiness that touches Brahman (brahmasaṃsparśam): that of which the contact is with Brahman the supreme, that is 'brahmasaṃsparśa' happiness; atyantam: it abides having passed beyond every limit, thus 'atyanta' — the most exalted, the unsurpassable — he attains (aśnute), that is, he pervades.