C50 Reducing Yang into Abdomen

What did it mean to shrink back into one's stomach? Master looked at me and asked, "Little kids shouldn't ask so much. I'll tell you in a few years." What does that mean, what is there to be ashamed of, to not tell it to me, I focused on Xia Yu.

Xia Yu's face was in pain, both of his hands seemed to be grabbing onto something inside the blanket, and not long later, he started sweating profusely