This is a duct for PLA-style cooling fan to be mounted on the back of the extruder stepper. Takes a 50mm fan. It directs the air under the stepper to point forwards towards the nozzle. v001 hangs too low to use with the stock heated bed, because it crashes into the bed screws. Use v002 if your printer is using the stock bed. v001 would be better and allow more air flow if you happen to have a flush bed. It'll need some help printing the bridges. I had a fan inside my case and it did a great job. Intended to be printed with the big flat side down. Right now it just kinda snaps on the back of the stepper motor, but I wasn't able to test it to see if that's secure enough. I honestly don't even know if this has any effect on the print. I'm just posting it here so other people can play with it. I ran the wires up with the rest of the extruder's wiring, and mounted a switch on the case to control it. Should be left off for first layers, and then turned on. All in all, it would definitely be better to make something that uses a centrifugal blower fan - something that has more pressure and can direct the air through a small duct like this better. But I had the 50mm axial fan on hand, so that's what I used.