Found this on the Ohlins website.
The spring pre-load affects the ride height, it does not affect the spring stiffness. Therefore, on models with a linkage to the shock absorber, the suspension may actually feel harder when you reduce the pre-load and the shock absorber gets into the harder range of the link system.
Also found this http://fixbroke.tumblr.com/post/39188918110/spring-preload-and-sag-what-does-it-really-do
Decent explanation of preload