Somebody wrote this yesterday. I can`t remember who !?
The more preload you add, the more the car will behave like with a locked diff during the change from being "on power" to "engine braking" (or the other way round). A clutch pack LSD without preload completely opens up (read: it's like a open diff) if there is no torque acting on it, which can have a very bad effect if you were relying on the stabilizing effect of the LSD.
So, it mainly affects you during the short time after braking for a corner, where you're almost not on the throttle yet (neutral load on the diff). If the car is oversteering in that situation, add preload. If it is too hard to turn and understeers off all the time, remove preload.
Phew, I thought I was the only one who didn't understand any of that 
