Toront suggested this is worthy of it's own thread so here it is. The warning in the arm is a good idea to obey, that thing has scary powers. Already posted this video but it explains it pretty well. Seen these only a couple of times, they aren't very common. Tracked vehicles are more common as in the image below, but as you see from the image below the 'arm' is similar. That arm grabs the tree from the base, cuts it, de-branches it and then cuts it into desired pieces and places them where you want. Scary efficient.