thumbscrew

noun3

1. •
   A screw that is designed to be turned with the thumb and fingers, thus requiring no screwdriver or spanner (wrench).
2. •
   An instrument of torture used to crush the fingers.
3. •
   (figurative) A weakness that can be taken advantage of.

   • Law 33: Discover Each Man's Thumbscrew ― Robert Greene, The 48 Laws of Power