hmmmm in the ten years I've been umpiring, I've never seen it done except in the set position. Just as Mr. Beck did. However YMMV. Than again, one must know the difference between the Wind-Up vs. the Set position, as well as the difference between pitching from the set position and coming set. Prior to coming set is where were discussing the gorilla arm movement. Again, YMMV.
Generally speaking, you can see the "gorilla" arm swing during the movement. There are times when a pitcher isn't swinging his arm that it can be difficult to see him come set from a baserunning perspective, depending on his personnal style. This is where the rule comes into play, as well as where a pitcher can obtain an advantage over the baserunner, by not being able to determine when he comes set.
I have seen pitchers using the wind-up that shake their arm and or hand, I don't really consider that a gorilla arm again..YMMV