I saw a nastier version of one of those on a Learning Channel show once. The big one could grab a tree, slice it off at the bottom, and then shoot it through a channel, whacking off all the branches and cutting off 12-foot sections on the fly, about one every second or two. It destroyed trees *fast*. Eek. Apparently, it was mostly computer controlled, the operator just told it which tree was the next victim and it did the rest.