This duct is closed and it transfers the air blown out of the PSU to the Y Stepper. Since the PSU is running much cooler than the Y Stepper this gives a fair cooling effect without additional fan, while protecting the PSU from falling garbage. I mounted my Y Stepper on the opposite side (compared to the original build instructions of the MM2 - note that this reverses the stepper direction). The original screws of the fan can be used - undo the inner screw, put on the duct, screw it on again loosely, then undo the outer screw (the duct can be rotated clock-wise far enough to access the screw), adjust the duct and snugly tighten both screws.