Bhashya
सापि कृतिः यदा सुखं लभते सुखं निरतिशयं वक्ष्य-माणं लब्धव्यं मयेति मन्यते तदा भवतीत्यर्थः ।यथादृष्टफलसुखा कृतिः तथैहापि नासुखं लब्ध्वा करोति ।
In plain words
That action takes place when a person realizes that unsurpassed bliss—described hereafter—is what must be attained. Just as ordinary worldly action is driven by the desire for visible happiness, so too spiritual effort is never born of sorrow.
Close to the Sanskrit
That action also, when one obtains happiness, thinking 'unexcelled happiness, to be described later, is to be attained by me,' then occurs—this is the meaning. Just as ordinary action has visible fruits of happiness, so here too one does not act having obtained unhappiness.