slacken off vs spin down

slacken off

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

  • To do (something) less intensely. 

spin down

verb
  • To diminish in energy; to slow down or peter out; to be gradually canceled or ended. 

  • To reduce its speed of rotation. 

  • To reduce its spinning speed from that required for reading and writing. 

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