Looser fins help the MirageDrive operate more easily. This is considered to be the best setting for the majority of users. Additional tension can be added by the user to tune-in their desired resistance or cadence. The looser setting is easier and generates better cruising speed in most situations. The ends of the thread are flattened and will cause friction to keep the nuts from backing off the threads. Loosen the nut until you reach this location and feel resistance, as demonstrated in the image below.
In theory a tighter fin is capable of generating more speed, but this requires more physical effort, so for most users in most circumstancesthis will bring diminishing returns
Watch this video for further information of tuning a V1/V2/GT or MD180 Miragedrive