Whenever human beings indulged in karma it created a karmic debt, the fructification of which can occur any given moment of life; now, a few days later even months or years later or after many manifestations by soul atman within. The karmic balance thus created continues.
We can undo the karmic balance; negate it in entirety by doing exactly the opposite of it. If we have a negative karmic balance of -45 then we can undo it by creating a positive karmic debt of +45. Indulging in good karma, positive karma or negative karma, bad karma is the game of life.
Every indulgence of bad karma creates a negative karmic debt and every act of good karma a positive karmic debt! To travel spiritual path successfully we have to negate karma to zero. The sum total of both the negative and positive karmic debts has to be reduced to zero.
This is what nishkama karma yoga all about. Indulging in karma, nishkama karma way of life does not produce any karmic debt negative or positive. A true seeker always indulges in karma as a trustee for whom fruits of karma indulged always belonged to the spirit, soul atman within.
Every action, every single thought produces a karmic debt positive or negative. By guiding our actions, thought processes we can indulge in good karma or bad karma any given moment of life.