Triangle escape boards are used under the supers/boxes that you want to get the bees out of. The triangle that is covered with hardware cloth, faces down. If your covers are propped up for ventilation be sure the openings are not so large that robbers can get in; or wax moths during the night. As the bees leave the super, it will be left undefended. I have used a fume board for a few [only 1 or 2!] minutes and then put the fume board on other hives that I am harvesting from that day that do not have the triangle escape.
The gadget: I don't know if they are used much anymore. The way they are constructed the little metal gates have to be set just right, otherwise the bees are reluctant to go through or they get plugged. The only thing good about them is that they are cheap compared to other escapes that you can make, or buy.
Addition: One thing about that bee escape gadget, one of the catalogues recomends using a shallow super underneath it as a spacer for the bees. I suppose an Imrie shim, formic acid pad spacer or one of those spacers for baggie feeders could be used also. A little cheaper than a complete triangle escape. It may work better that way if anybody wants to still use those gadgets.