Maybe it is some kind of mechanical device that can extend out 12 inches to protect passersby below from falling rocks. Otherwise, if "it" is an animal, it means that it will extend one of its legs and stop the rock with its foot.
The phrasal verb is "knock loose." It means to dislodge, or hit something so that it is no longer secured in place. Sometimes kids will fall off a bike and knock a tooth loose. They have to go to a dentist.