I agree with ewa. just to add some good points, we can also use inversion for emphasis: "NEVER should you underestimate me." is more effective rather than "you should not underestimate me." "LITTLE did she know that my father is an army." is more surprising to hear than "she didn't know that my father is an army." or to use inversion to emphasize conditional: "ONLY after you find the needle in the haystack can you pass judgment." If you happened to read some literary works, you will likely to notice these kind of sentence structure. :)

You're right-both sentences employ what is called inversion, for greater dramatic effect. In the first sentence "down"(a preposition) is put at the beginning of the sentence, which is often done in narratives. Other examples could be "Off you go!" or "We opened the box and out came a tiny animal." The other sentence begins with a negative word (nor),which requires you to use inversion in the following clause.( Inversion basically means using the question word order). Other examples of the same process including negative words: Not only was he talented but also really amiable. Compare: He was not only talented but also amiable.(-a neutral sentence,no inversion) Never have I seen such a huge building. Compare: I have never seen such a huge .. On no account can you touch this button. etc. Inversion is typical of more formal ,often written English (or "dramatic narrative") Other words which can be put at the beginning of the sentence with inverted word order include :Hardly/barely/scarcely or time expressions: hardly ever/rarely/seldom/not until etc. Hope this is helpful!