The preload on a clutch diff just gives it enough to not spin when one tire is unloaded. With both touching the ground it uses the ramps inside to add pressure as you accelerate and also controls lock on decel to steady the car under the brakes. Too much and it pushes like a pig (rear wheel drive) and too little it will wiggle and cause one tire to back spin and pitch the car. There is a lot more to them than just the breakaway torque.