I am not sure how this works?

The smaller big loop girth hitches to your tie-in place on your harness. Clip a locking biner into the big big loop-now you can clip this into your belay anchors, or into a gear loop on the side of your harness while climbing.The smallest round loop in the middle is where you clip in your belay device with a locking biner. This setup works great.You can see pictures and instructions on the Mammut website.
View Details: Mammut Belay Sling - 19mm >