when you cut down a spring you increase its stiffness and when the rebound spring gets compressed the total stiffness (main spring + rebound spring) will be higher and the rebound travel shorter.
If you have a clearance at static ride height for the rebound spring the stiffness is only due to the main spring and is lower.
If there is no clearance, both springs being compressed, the preload doesn't change the stiffness but only the ride height and the point where the rebound spring becomes free.
SSL front suspension kit solves the problem by adjusting the preload of the main spring independently of the rebound spring
which is free at static ride height.
Unless I am corrected by Dan White or Peter Ballard I'm going to suggest that the above is not quite right.
The threaded adjustment on the SSL is not there to adjust the pre-load it is there to adjust the ride height so that the rebound spring can be set accurately to just kiss the hub at the static ride height.
Admittedly, if the rebound spring is already in compression, it will adjust the pre-load but in this condition the main and rebound springs will be adding to each others rate and the ride will be very harsh.
The idea with the SSL is that you determine the ride height you want with the spacers in the kit then adjust the threaded seats to just get the rebound spring to turn at the static ride height.
To get the extra ride height relating to sump clearance the spacers go in with the rate riser spring above the stub axle and to get a lower ride height the spacer goes between the rebound spring and the stub axle.
The total difference in ride height in the standard kit between the 2 spacer settings is 20mm.
The big benefit with the SSL is that you can get the relationship between the main and rebound springs set perfectly ( albeit with a bit of experimentation and readjustment ) without having to chop the rebound spring.
Chopping the rebound spring is not a good idea as mentioned above so the only other way to get it spot on is to have a variety of rebound spring lengths and keep changing them until the clearance is perfect. Of course as the suspension settles or the main spring sets a little with time you would have to change the rebound spring whereas with the SSL you can simply adjust the threaded spring seat to suit.
I guess you could just start with a shorter rebound spring and shim it to get the correct clearance but that would be a real pain and you are starting off with a short spring in the first place which is a bad idea.
Added to that the rate riser spring helps to prevent bottoming during hard sudden bumps.
As you can tell, I'm a big fan of the SSL although you do need to understand it and play with it a bit to get it perfect.
If Dan or Peter are watching, I stand to be corrected.