大乘起信論義記 3.480

3.480

謂先離惡業，後滅惑障。業中離誹謗等不起新業也。重罪漸薄者，重業輕也。

This means that first they separate from evil karma, and later they extinguish the hindrances of affliction. Within karma, separating from slander and the like means not giving rise to new karma. Grave offenses gradually thinning means that grave karma becomes light.