This (abridged) entry from Swan in Practical English Usage (p303) under the heading Inversion after negative and restrictive expressions provides the answer to your questions:


  If a negative adverb or adverbial expression is put at the beginning
  of a clause for emphasis, it is usually followed by auxiliary verb +
  subject. These structures are mostly rather formal.
  
  
  At no time was the President aware of what was happening. 
  Seldom have I seen such a remarkable creature. 
  Little did he realise the danger he faced.
  Only then did I understand what she meant. 
  Not only did we lose our money, but we were nearly killed.
  


Here is a short Wikipedia article on inversion.

Question-like structure (word order) in non-question sentences
I'm looking for a comprehensive reference to  existing proofs of Rokhlin's theorem that a 4-dimensional closed spin PL manifold has signature divisible by 16.
I'm specifically interested in direct proofs (if any such exist) which do not rely on the fact that $\pi_i(PL/O)=0$ for small $i$.

The most commonly cited reference seems to be the book by  Kirby "The Topology of 4-manifolds". But the proof there is for smooth manifolds and I'm not sure why it works for PL manifolds although I've seen it claimed in various places that it does. The same is said about Rokhlin's original proof but I don't know why that's true either. I would also like to know if other proofs for PL manifolds exist. I'm particularly interested to know if there is a PL proof based on the Atiyah-Singer index theorem.       
