It's a big beam, say 8" square by 8' long or longer. Each end is held in a cross piece, one end higher than the other. You put the piece to be planed on top and plane down hill. If you need to, a couple of screws in the lower end will hold the piece in place. You can also nail a long thin board to the side and use that to hold a piece as a sticking board. Let gravity be your friend. Here's an example: http://images.search.yahoo.com/image...p3ulg&fr=aaplw
PS, I like them a lot, so I installed a smaller (say 4" square) version with a bench slave on the back side of my bench. Works great.