Andrew Wilkinson wrote an answer about Mad Rock Mad-Lock Belay Device on November 27, 2009
its just like a regular atc.
you take a bite in the rope put it in the belay device and attach the carabiner. then when your belaying someone take slack in accordingly and if they fall quickly pull the rope down to lock it. please use professional instruction to learn. dont take my word for it but thats basically how you do it