Usually it's a dildo connected to a long rod, which is connected to the outer edge of a rotating wheel. As the wheel rotates, it shoves the dildo in and out of the girl - kind of like a pumping action. Some others are connected to hammer drills and such tools.
Some look sort of half-made to me - like an accident waiting to happen. A girl would have to either have some major courage, or be really drunk, to be the first to try a new one out. I figure there's at least a few girls out there who probably don't exactly have a social life anymore after a mishap. :eek: