This is a holder/adapter to attach a 40x20mm blower fan to E3D and compatible hotends.
The blowers output air beam is split in two so that part of the air cools the hotend and the other cools the print.
THE BLOWER HAS TO BE ON ALL THE TIME in order to keep the hotend cool.
Please mind the dimensions (see pictures) before printing to see if it fits your setup.
If you are a curious maker / "tinker" / hacker, have a look at my blog: