constrain vs let off

constrain

verb
  • To force physically, by strong persuasion or pressuring; to compel; to oblige. 

  • To keep within close bounds; to confine. 

  • To reduce a result in response to limited resources. 

let off

verb
  • To forgive and not punish. 

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

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