Karma in its effect on character is the most tremendous power that man has to deal with. Man is, as it were, a centre, and is attracting all the powers of the universe towards himself, and in this centre is fusing them all and again sending them off in a big current. Such a centre is the real man — the almighty, the omniscient — and he draws the whole universe towards him. Good and bad, misery and happiness, all are running towards him and clinging round him; and out of them he fashions the mighty stream of tendency called character and throws it outwards. As he has the power of drawing in anything, so has he the power of throwing it out. (I, 29-30)
Swamiji is revealing here the secret that every work which we do, creates its specific vibrations at the cosmic level, thereby reminding us to be alert and aware of each and every action we perform. Good and bad, happiness and misery are all created by ourselves. The work which we are destined to do has direct connections with the births which we will have to take.
Karma Yoga opens up the way to eradicate the dirt and pollution which we go on accumulating in our minds through our work. Sri Krishna reveals in Gita that through repeated good actions, our good samskaras get accumulated through repeated births, and finally the Yogi acquires real knowledge and realises the Truth.