They look like a Torx screw head turned inside out. Imagine something between a pan head and a round head (ie slightly domed) with six small notches in the edge. The sides are sloping/curved and they're usually hardened, and the edges of the notches are slightly rounded off so you can't get a grip with pliers. There's a picture at http://website.lineone.net/~dave.cushman/zeroscrew.jpg