Rimpoche explained in his example of the warp and woof of woven cloth how causes and conditions intersect to produce one's experience of the result.  He also mentioned how purification using The Four Powers helps to mitigate the conditions to the the result of negative karma is not as severe.  He also mentioned that dedication helps preserve the results of positive karma.  This excerpt from Rimpoche's transcript might help explain the process of purification:

Purification is basically the four powers. The first is the power of the base. You created negative actions. You take refuge in the enlightened beings and you generate compassion for non-enlightened beings. That provides what is called the power of the base.

The second is the power of regret. If we don’t regret our negative actions we’re not going to be able purify them at all. Why should we? If I did nothing wrong, why should I have to engage in purification? Last night on TV I saw the arrest of an accused pedophile who kept on saying, “I did nothing wrong.” If you don’t believe you did anything wrong, you have no reason to purify.

If there is no regret, there is no repentance. When you have regret, then you have to take physical, mental, and emotional action. [These are the power of antidote action.] Mental action is actually more important than physical. Mental action is regret and the power of non-repeating, promising not to do it again. Generating bodhimind and meditating on emptiness are also mental actions. They are the most recommended mental actions.