There are two metal "rods" holding it in place.  Just bend them down and out, and the bag will slip right off.  It takes a bit of muscle to get 'em started, and it feels like you're going to break them off, but that's how it's done.  You can feel them when you feel underneath the plastic bag topper, and they meet in the middle (so the "hinge" is on the edges, if that makes sense).  Terrible design if I do say so myself.