Krishna talked about yagna (selfless actions) at two places (3.9 to 3.15 and 4.23 to 4.32). He cautions (3.9) that motivated actions bind us (karma-bandhan) and advises action to be performed without attachment. He further indicates that the selfless action of yagna holds the supreme power (3.15) and in the beginning, using this power the creator created (3.10). He gave many instances of yagna (4.23 to 4.32) and concludes that all of them are offsprings of actions and this realisation would liberate us (4.32).
This is the Lord's assurance on liberation.
Furthermore, about sin, Krishna indicated (2.38 and 4.21) that action arising out of an imbalance between polarities of pleasure-pain; profit-loss; victory-defeat is the sin that results in karma-bandhan of perpetual guilt, regret, grudges and hatred. He further said, "One hoping for nothing and doing mere bodily actions, incurs no sin" (4.21). He concludes that by sacrifice (of attachment) the knowers have destroyed sins (4.30). This is an assurance from the Lord about the destruction of sins in yagna.
Krishna earlier declared (2.47) that we have a right over karma but not over karma-phal (fruits of action). Here he reveals a secret (4.31) that the remnant of selfless action of yagna is the nectar of Brahman (Supreme God). The subtle indication is that whatever we are ever blessed with is a result of the selfless actions we perform consciously or unconsciously. Another inference is that if we take contentment as the parameter, the rich become richer and the poor poorer. This is an assurance from the Lord about the nectar of contentment from yagna.
It is the motivation that makes an action a sin and when the same action is performed like yagna, it becomes virtue which is nothing but the nectar of liberation and contentment.
Source - Daily World