(my attempt translation) Document shows Nayin started from Tang dynasty Lei family... at the area on the top board facing two sound holes, the wood slightly swell, as leek leaves. Sound wants to come out but confined a little bit, lingering around, therefore creates more subtle sound.
although some qin's Nayin is curved in instead of curved out. This picture is my sketch of one of my qin made by Wang Peng.
From handed down ancient qins, one can see their Nayin has different heights. Since several hundred years of wear and tear, some has the Nayin implicit rather than explicit... but some qins has high nayin, swell pretty high, only 5 fen (about 1.5cm) to the bottom. This kind of high nayin is suitable for qin with thinner top board.... some qin has no nayin, but has very thick top board and thin edges, which actually is doing the same function as nayin...
This is interesting Nayin. It could fits the "隐而不显"implicit rather than explicit "面板很厚而边薄" category that the top board is thicker in the center and thinner towards to the sides.
中文資料來源 Chinese Article Source: