Franc Otte,
Just in case Gunman misses your post, I will take an educated guess that he refers to a vice where the jaws are hinged to each other well below the the level of the bench. This gives them a scissor action as opposed to the parallel action of a normal engineering vice.
A similar system is used to join soft jaws for a gunsmith's vice, essentually so the jaws can be inserted and removed from the vice as one piece as circumstances dictate. They are often sprung apart so to open and close with the vice without having to be attached to the jaws.
I appologise if I have got this wrong and await correction.
