knock on wood vs throw off

knock on wood

verb
  • To take a customary action to ward off some misfortune that is believed to be attracted by a presumptuous statement. 

intj
  • Hopefully; a self-directive to undertake the customary action to ward off bad luck. 

throw off

verb
  • To introduce errors or inaccuracies; to skew. 

  • To expel, reject, or renounce. 

  • To give forth in an unpremeditated manner. 

  • Of a horse, to eject its rider. 

  • To confuse; especially, to lose a pursuer. 

How often have the words knock on wood and throw off occurred in a corpus of books? (source: Google Ngram Viewer )