Matt M wrote an answer about Mammut Belay sling Polyamid Sewn Runner - 19mm on November 13, 2010
So I haven't USED one in person but I had something like this BITD. You Girth Hitch the largest loop to your harness ala a P.A.S. or Chain Reactor. So this is just a guess.
For Belay, the smaller loop on the other end is clipped into your anchor "power point" or what have you. You then use the middle, smallest loop to belay off of using a guide style/ auto BLOCK belay device (ATC Guide, Reverso 3 etc etc) or GriGri.