It was made of four 'leaves' which joined together by a hinge at the top. A screw inserted between the leaves could be turned to slowly separate the four lobes of the 'pear', expanding the orifice into which it had been inserted. The torturer could choose whether to simply distend the cavity, or expand the pear to it's absolute maximum and mutilate the prisoner.
Of course, the orifice which was on the receiving end of the pear was symbolic of the offense - heretics received the oral pear, passive male homosexuals received the anal pear, and women who had intercourse with Satan were in receipt of the vaginal pear.
These devices were much more intricately decorated than many other tools of the trade, bordering on works of art. It is doubtful, however, that the embellishments made much difference to anyone who experienced the torture of the "Pear of Anguish".