I've not used one, but I have previously read a few threads about them in various places, and the basis involves a working table with a grid of holes and/or slots.

Place what you're working with onto the table, then block as many of the holes/slots that the work doesn't cover. This can be done simply with scraps, or via more elaborate methods, such as plugs for the holes, or valves that cut of vacuum to specific parts of the table.

The most interesting method I seen though, was to have the above grid, but cover the entire thing with a sheet of MDF. The MDF is porous enough let the vacuum through and hold down quite alot of things without needing to worry about blocking areas, and is sacrificial so that if you do have any cutter/table incidents, you can just replace it. I can't remember where I seen that discussion though!