Bhagavad Gita Verse 25, Chapter 17

tadityanabhisandhaaya phalam yajnyatapahakriyaahaa |
daanakriyaashcha vividhaahaa kriyante mokshakaankshibhihi || 25 ||
 
With (chanting of) Tat, without aiming for reward, are the various acts of sacrifice, penance and charity performed by the seekers of liberation.
 
tat : Tat
iti : in this manner
anabhisandhaaya : without aiming for reward
phalam : reward
yajnyatapahakriyaahaa : acts of sacrifice and penance
daanakriyaahaa : act of charity
cha : and
vividhaahaa : various
kriyante : performed
mokshakaankshibhihi : seekers of liberation
 
Karmayoga suggests that we should perform our duties in order to exhaust all of our vaasanaas, our latent desires, so that our mind becomes pure and fit for spiritual advancement. The primary obstacle in karmayoga is attachment at two levels. We are attached to the personal reward of an action. We are also attached to the act itself. For instance, a musician may become attached to the royalties promised to him in his recording contract. He may also become attached to the unique style of music that he has developed.
 
Shri Krishna says that chanting the word "Tat" when performing any saattvic action has the effect of removing these two kinds of attachment. Tat means "that" in sanskrit. It is a pointer to the eternal essence in its transcendent aspect, also known as Ishvara. Using the word Tat, we can dedicate our actions and results to any deity that we like. The act is performed by Ishvara, therefore the reward goes to Ishvara. I am only the instrument of Ishvara. This is the attitude developed when Tat is chanted with faith.
 
How does this work? Imagine that you are going overseas for closing a business deal. If you go with attitude that you am doing this deal for yourself, then you have to take on the associated stress that comes with the deal. But if you go with attitude that you are closing the deal for your boss or your company, your stress will significantly reduce. You are just carrying out instructions given by your boss. If the deal works out, great, otherwise there will always be another opportunity.