improve vs twist the knife

improve

verb
  • To become better. 

  • To make (something) better; to increase the value or productivity (of something). 

twist the knife

verb
  • To deliberately do or say something to worsen a difficult situation or increase a person's distress, irritation, or anger. 

How often have the words improve and twist the knife occurred in a corpus of books? (source: Google Ngram Viewer )