Usually the thumbscrews consisted of two flat metal pieces separated by one or more screws. The torturer could use the device as a restraint - like handcuffs - or increase the pressure and destroy the digits placed therein. Simple, yet brutally efficient.
This device was found in Europe, India and China in one form or another, and it is easy to understand why. Crushing fingers, thumbs and toes was an easy way to inflict maximum pain with minimal effort.
Trivia & Comment
Many thumbscrews that still exist are considered elaborate works of art, and it is evident that the blacksmiths who forged these pieces put great pride in their work.