Well, it doesn't really "slow" the servo movement, but it brings it closer to a linear or exponential movement corresponding to how much you turn the wheel.

What the expo then does is change the servo travel to steering wheel travel exponentially. What this does is make it so if have negative expo your turn the wheel 100 percent, the servo will travel 100 percent as exponential doesn't change the endpoints, but then 50 percent turning the wheel may be just 35 percent actual wheels turning, and 25 percent steering input may result in just 10percent the wheels actually turn.

In simpleton terms, negative exponential makes the car smoother to drive and smooth outs initial steering input, but at the edges of steering input the car will feel twitchy.

Positive expo will create a very responsive car around center, one that is rather twitchy, but at close to max steering the inputs will smooth out.

Most people run between 10-20% negative expo as it makes the car easier to drive around neutral steering, where much of the steering needing fine input adjust is needed.