Variations were in existence - for example, the "toe-breaker", in which the screw was used to drill holes into the foot; or the Venetian "foot-screw" which held rows of spikes embedded in the upper bar.
A device akin to the thumbscrews, the foot press was designed to (un)comfortably fit a single foot between a sharp, ribbed iron plate and a bar of metal attached to a long screw. When the screw was turned, the bar crushed the foot into a mangled pulp.
Trivia & Comment
Believe it or not, more than a few foot fetishists are fascinated with the function of this fearsome phalange crusher.