An hua means "hidden" decoration. It is un-colored and only seen as impressions under a clear glaze or through transmitted light. The decoration could be produced either by carving, incising or impressing the design into the porcelain before glazing and firing. The decoration was first seen as the dominating decoration on Song (960-1279) dynasty Ding and Xing wares and continued on Qingbai wares, under a slightly bluish glaze.
Hidden decoration was very popular in the early Ming period, at least in part because of Yongle's taste for plain white wares. Several reason's have been proposed to explain Yongle's preference for white wares, which represented quite a change from the taste for brightly colored underglaze blue and copper-red wares of the late 14th century. Since white is the color of filial piety and mourning, one of them is that Yongle's choice was partially intended to mitigate the circumstances under which he came to rule, which also caused him to spend the first four years of his reign in mourning.
Hidden end carefully engraved thin lines or decorations was also a key feature in borders on "high transitional" pieces around the 1640's. The technique was revived as in cut and impressed rim decorations in the Qing Dynasty during the first half of the 18th century after which it develops into a colored surface-covering version called "graviata".
Decorating with painting in white slip on a white ground are by some authorities also regarded as anhua, which at least to me is not correct. See further: "Bianco-sopra-bianco".