Im gonna be devil's advocate...
Bottom perch/adjuster nut type coilovers: correct me if Im wrong, if the spring is fitted to the shock and the slack taken up by the adjuster nut, the shock will be fully extended. Iow, if a spring is anchored and tensioned it will have an opposite force, meaning it will extent the shock fully.
That said, by further pretension of the spring the spring will get shorter but the same principle remain and the spring will still push the shock to its fully extended length. How can the car lowers itself when the spring is tensioned?
I can understand that if the car sits to low and the bound/ rebound position on the shock also to low/little, pretension can get the car higher because coil springs used are linear example; 1" = 200lbs, 2" = 400lbs etc - Because pre-load doesn't increase spring rate at all, all it does is add a force that has to be overcome before the spring takes action It seems pre-load is a nasty thing...
And this begs my question, how would the adjusting of the spring (compressing/releasing) raise or lowers a car
EDIT: I was lucky with my spring rate - the car squat down to the designed ride height and the springs feels nicely weighted and act immediately when putting further weight on them. But I sat for quite awhile looking at the front to see if I tension it, there would be a difference but fail to. Not keen adjusting them under load of the car to see if it will. Btw, I only pretension my springs by hand, about half and inch before installation
I think the art is rather to have the springs calculated correctly that when the car is loaded on them, it sit on its desired height.