backtrack vs retrocession

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. 

retrocession

noun
  • The act of retroceding; a going back. 

  • The transfer of risk from one reinsurer to another. 

  • The return of land, rights, etc. previously ceded. 

  • Metastasis of an eruption or tumour from the surface to the interior of the body. 

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