Wu YinandSi Hu, orFive YinandFour Hu,are terms for Chinese opera. They are about the proper pronouncing skills.
Yinmeans note in pronunciation.FiveYin,the five notes of the ancient pentatonic scale, refers to the five places where airstreams pass through to make different sound. It includesHou(throat)Yin,Chi(through teeth)Yin,Ya(teeth)Yin,She(tongue)YinandChun(lip)Yin.
FourHu, referring to the proper shapes of the performer's mouth when pronouncing, falls in four categories --Kai Hu, Qichi Hu, Hekou HuandCuokou Hu.
Only when the performer gets full understanding and control ofWuYinandSi Hu,can he make the proper and articulate pronunciation when performing.
Nowadays, when we say someone isWu Yin Bu Quan (an idiom), we mean that this person is not good at singing at all.