clench vs let off

clench

verb
  • To close tightly. 

  • To grip or hold fast. 

noun
  • A tight grip. 

  • A local chapter of the Church of the SubGenius parody religion. 

  • A seal that is applied to formed thin-wall bushings. 

let off

verb
  • To forgive and not punish. 

  • To cause to explode or come out; to release. 

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