Visuomotor is the coordination of movement and visual perception by the brain. And a Visuomotor Gain system scales the rotation linearly in order to maintain better consistency and perception when rotating your view.
To break it down further, let's start with an analogy. When you look at an image and move it around with the mouse, you normally expect the same physical distance on the mouse to always move that image by another constant physical distance on your monitor. When you zoom in on that image, you also expect the visual relationship between those distances to remain the same.
In Siege, the image is actually what you view through the player's camera, and "moving it around" in that context means "moving the 'panoramic view' around". Once again, regardless of the zoom, a given physical mouse or gamepad stick distance will always match another physical monitor distance and scale linearly. So in practice, the more you zoom in, the less the camera will rotate in degrees, therefore automatically increasing the 360° distance.
Because of the consistent feeling visuomotor gain provides across all zoom levels, we decided to use it as the new ADS sensitivity model in Siege.