The word "翻天覆地" is formed by combining two parallel verb-object phrases: "翻天", meaning "overturn the sky", with "翻" as the verb and "天" as the object, and "覆地", meaning "cover the earth", with "覆" as the verb and "地" as the object. These phrases are joined to create a compound idiom where each part reinforces the other, emphasizing a total and drastic change.