I'm confused with grammar of the next sentence. It is from "harry potter and the prisoner of azkaban".
"Shards of glass flew in every direction and Aunt Marge sputtered and blinked, her great ruddy face dripping."
The sense of the sentence is clear but could you please explain me the last part 'her great ruddy face dripping.'? It sounds for me like it's missing something else here and I can not find any grammar rule for this case.