This mod dynamically scales mouse sensitivity when aiming down sights with weapons.
The aim sensitivity in your settings menu is ignored completely. Instead, this mod uses your default FOV, look sensitivity, and the weapon's current ADS FOV when calculating the new sensitivity. If you move your mouse a 1/4 inch, your crosshair should move roughly the same amount of pixels on your screen regardless of how zoomed in you are.
For more information on the math behind the sensitivity scaling, please visit https://themeta.com/converting-sensitivity-between-fovs-focal-length-scaling/