With the head placed under the upper cap and the chin placed above the bottom bar, the top screw was slowly turned, compressing the skull tightly.

 

First the teeth are destroyed, shattering and splintering into the jaw. Then the eyes are squeezed from the sockets - some versions had special receptacles to catch them. Lastly, the skull fractures and the contents of the head are forced out.

 

In earlier stages, the torturer could keep the head firmly clamped and strike the metal skull cap periodically; each blow echoing pain throughout the victim's body.

There is an excellent photo of a head crusher on the cover of Born Annoying by Helmet.