To become like a rotten stump means first of all to become like a tree that has been cut down. A tree that has been cut down is completely stripped of its trunk, branches and leaves. It can no longer take in energy from the sun. So this means to cut off whatever gives energy to your small self.
Sometimes you see a tree stump in the forest with new branches and leaves growing from it. This is because the roots are alive and continuing to draw nourishment from the soil. So if you want to attain your true self, you must become like a rotten stump, which means to let your roots die so that you stop feeding your small self.
Finally, a rotten stump nourishes the whole forest. Insects, reptiles and small mammals take refuge in it, and they feed each other and the birds and other animals. As the stump slowly decomposes it releases all the nutrients that were bound up in it and those leech through the earth enriching the soil, helping other plants to grow. This means that your practice is no longer for yourself but rather for all beings without discrimination.