drop off vs spin down

drop off

verb
  • To lessen or reduce. 

  • To fall. 

  • To deliver; to deposit or leave; to allow passengers to alight. 

  • To fall asleep. 

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 drop off and spin down occurred in a corpus of books? (source: Google Ngram Viewer )