backtrack vs hark-back

backtrack

noun
  • the act of backtracking 

verb
  • To retrace one's steps. 

  • To exercise a racehorse around the racetrack in the opposite direction to that in which races are run. 

  • To taxi down an active runway in the opposite direction to that being used for takeoff. 

  • To repeat or review work already done. 

hark-back

noun
  • An act of alluding, returning, or reverting (to a subject previously mentioned, etc.); also, an act of evoking, or longing or pining for (a past era or event). 

  • An act of hounds retracing a course in order to pick up the lost scent of prey. 

How often have the words backtrack and hark-back occurred in a corpus of books? (source: Google Ngram Viewer )