The screwdrive slots are the damping adjustment ie they control the way in which the spring behaps as it compresses and extends. The first adjustment you carry out for extra load will the the spring preload, to do so, you'll have to slacken the lock nut and turn the body.
Once set, you'll need to adjust the forks to compliment the rear setting.
moto.racing.co.uk will give you an overview of what you're trying to achieve but the Buell rear shock will physically behave in reverse.
If it's just the odd ride and you're not going balls out, then I'd just adjust the preload since the X1 shite; get an AST shock for Xmas, your X1 will be like a different bike
