The foundation meridian was the foundation of energy. It was the one closest to the navel, the first and the beginning point of blood and meat. Within the human body, it was the source of all energy.
When one trained the foundation and breathing meridians, one was solidifying and training the source itself.
When one trained it to the extreme, one could return back to birth, like those babies who weren't born, breathing through their navels, connected to heaven and earth.
When the bits of true qi entered his abdomen, Zong Shou started to use his strength, shaking the muscles around his abs, trying to get each and every bit of true qi to seep into his body and muscles.
Although Little Green could purify a lot of true qi, there was a limit. Zong Shou wasn't greedy, absorbing around 60% of it before letting go of his hand.