let up vs sit back

let up

verb
  • To cease or stop (doing something intense). 

  • To cease or stop. 

sit back

verb
  • To relax, to not exert oneself. 

  • To recline while still in a seated position, with one's back on the frame of the seat. 

How often have the words let up and sit back occurred in a corpus of books? (source: Google Ngram Viewer )