That is the development of thought in the Gita, that, by a certain spiritual technique, you convert action into inaction: you get the fruit of inaction through action itself. There is such a state of mind, calm and steady, in which you do a lot of work, yet you don’t feel it, you carry so little burden.

यस्त्विन्द्रियाणि मनसा नियम्यारभतेऽर्जुन। कर्मेन्द्रियै: कर्मयोगमसक्त: स विशिष्यते॥७॥

Yastvindriyani manasa niyamyarabhate arjuna; Karmendriyaiḥ karmayogamasaktaḥ sa visishyate—3. 7

‘But, O Arjuna, one who, controlling the senses by the mind, and remaining unattached, directs the organs of action to the yoga of action, excels.’

Sa visishyate, ‘he or she excels’. Who? Indriyaṇi manasa samyamya, ‘who disciplines the sense organs by the mind’. Mind is the master. arabhate arjuna karmendriyaiḥ karmayogam, ‘O Arjuna, he or she is busy in the yoga of action with the sense organs of action’; such a person does it with a special yoga buddhi; that karma becomes karma-yoga, not mere action, but action which can elevate a person spiritually. Asaktaḥ, ‘unattached’. The mind is not at all attached to these things. It has been detached from these things. Therefore, it can do things better and better. Asaktaḥ sa visishyate, ‘such an unattached person is excellent’.

Here comes human excellence in action. This particular verse is the best description of excellence in action. Niyamya, ‘disciplining’, indriyaṇi, ‘sense organs’, manasa , ‘by the mind’, arabhate karma-yogam, ‘does the yoga of action with that attitude’; such a person is a karma-yogi . Sa visishyate, ‘he or she is most excellent’; that is the training everyone has to give to the mind. A sense of masterliness is manifested there. One is not lost in the work, but is always free. Work with freedom, that is the idea.

Reference: Universal Message of the Bhagavad Gita Vol. 1 by Swami Ranganathananda (p.259,263)