Krishna says that the yogis, having abandoned sangam (unity/attachment), perform action merely by the senses, body, mind and intellect for self-purification (5.11). It is interpreted that even though one abandons attachment in the present moment, his past karma-bandhan (bondage of action) needs to be attended to. Hence, he keeps performing actions. Another way to look at it is that once attachment is abandoned, there is nothing left for him to attain in the physical or manifested world and all actions lead to purification of the inner self.

Krishna further says that the yukta (balanced/steady) having abandoned the fruits of action, attains eternal peace whereas the ayukta (imbalanced), impelled by desire and attached to fruit, is bound (5.12). One of the foundational pillars of the Gita (2.47) is that we have right over karma (action) but not karma-phal (fruits of action). Abandoning the fruits of action means that there is no preference for any specific outcome as one is ready to accept any outcome, however wonderful or frightened, with equipoise. Krishna earlier (2.66) said that the ayukta lacks both buddhi (intellect) and bhaav (emotions) and as a result, he will neither get peace nor joy.

Krishna continues that mentally renouncing all actions and self-controlled, the embodied rests happily in the city of nine gates (physical body) neither even acting nor causing to act. (5.13)

The key is mentally renouncing all actions even while acting or becoming a cause for an action. Karmas keep happening whether we are doing them or not and we merely become a part of them. After we consume food, hundreds of actions happen before it becomes a part of us and we don't have any clue about them. In fact, miracles like digestion happen when we are not a part of them at the mind level.

Source - Daily World