Hmm. Hang on.
Let's say the bike is on a workshop stand, forks fully extended, and the caps get wound in. This is shortening the spring length, isn't it, by reducing the space available inside the fork leg? And isn't the outcome of that putting more pre-load on it?
The effect, when the bike comes off the stand, is to alter ride height...
OK, letís first go back to one of my comments Until the fork is at full extension, or at least until the top out springs begin to have effect, the adjusters are purely ride height adjusters and nothing else but.
Next, letís imagine a reasonable build. Letís suppose one puts in a set of new springs, and for purposes of discussion, makes a spacer that will result in 10 mm of Installed Preload existing with the Front End Ride Height Adjusters fully backed out.
Next we re-install the front forks, put on the wheel, etc. and get the bike off the stands so it is resting on the shop floor nicely balanced by you straddling the bike or holding a bar end.
The front end will be in a state of equilibrium re ride height. Letís pretend you get a Free Sag of 15 mm. Keep in mind that the front spring will have settled upon a certain length, the number of which is irrelevant, aside from it being substantially less than the spring length when the forks were off the bike and in your shop.
Letís suppose you want less Free Sag because you want less Rider Sag. So you screw in your adjuster 2 turns which translates to 4 mm of change. You are in the travel zone of the fork, and what happens is that the steering head of the bike is lifted up by 4 mm (ignoring the trigí re the steering head angle just to keep things simple) as the fork tube is forced out by 4 mm as the adjuster lifts up the steering head upon the spring, while the spring length itself remains the same. Because the springís compressed length has not changed, the preload has not changed, and aside from some weenie weight transfer to the rear, the weight of the bike that the front wheel sees has also remained the same.
Therefore, the adjustment accomplished is front (chassis) ride height. The amount of resistive force in the springs is still the same, the compressed spring length is still the same, as is also the front tire contact patch weight upon the floor. More preload has not been applied.
And if any doubters remain, have a look at Andrew Trevittís excellent book Sportbike Suspension Tuning. Itís not just me singing this song !