The leash attaches to the belt on a sliding snap hook. It's not locked into position, but it also won't slide 360 degrees around your waist. The reason for this is that the belt buckle and tri-glide (the bit that allows you to adjust the size of the belt) prevent the snap hook from sliding further on the belt.
We do think that the ability to slide on the belt is desirable, at least with the dogs we've tested. This allows the dog to move from one side to the other without you having to adjust the belt.