Life is a two way process. We receive various stimuli and we keep responding to them. In this context, Krishna says that the knower of truth, united with self, thinks that, "I do nothing at all even while I see, hear, touch, smell, eat, walk, sleep, breath (5.8) - talk, evaluate, accept and even my blink of eyes and the senses move among sense objects" (5.9). In this existential verse, Krishna is describing the peak experience of the knower of Truth.
As we regularly experience emotions arising out of praise and criticism, we see that praise makes us forget ourselves like the proverbial crow dropping its meat upon hearing praise from the fox about its singing ability. Likewise, when criticized, our response varies from silence to verbal to physical depending on the degree of criticism and the strength of the critic. We assume these stimuli to be true and we identify with them. This leads to misery especially when we take them personally.
Our sense organs are like modern day electronic instruments which automatically respond to external stimuli like the ear to sounds and the eye to light, which is essential for survival. Response to the external stimulus can be automatic or controlled by us. Living in ignorance is being reactive where responses to stimuli are mechanical. However, with awareness we can control our response by realising the nature of senses which are mechanically drawn towards sense objects. It is essentially being proactive.
The obstacle is our identification with stimuli like praise and criticism, which create scars of karma-bandhan (bondage of action) lasting for an entire lifetime. Hence, Krishna advises us to realise that senses are mechanically interacting with sense objects and ‘I’ do nothing at all. This realisation is nothing but transcending from kartha (doer) to sakshi (witness).
Source - Daily World