slacken off vs wind down

slacken off

verb
  • To slacken or lessen; to become less intense. 

  • To do (something) less intensely. 

wind down

verb
  • To slow, as if coming to an end; to become calmer or less busy. 

  • To unwind. 

  • To lower by winding, as with a crank or windlass. 

  • To relax; to get rid of stress; to destress. 

  • To shut down slowly (by degrees or in phases). 

How often have the words slacken off and wind down occurred in a corpus of books? (source: Google Ngram Viewer )