tell me about the needle valves that are supposed to be in this extruder of theirs. ive been watching this machine for a long time. how i understand it is the needle valves close off the hot end while not in use or during a travel move so there is no ooze or wiper wall needed. on the other hand i have heard people say the valves arent what you think they are. not sure what that means so please enlighten me on this feature.