Kirk
The limb cores are a hi-tech material I know very little about. Bob had the stuff made up because he had trouble finding what he wanted. I believe that most olympic limbs are using foam cores now. My vague understanding is that it's small bubbles in some sort of medium. It is light in weight, very consistent, and durable. These limbs are very smooth and stable, and I believe the lighter weight increases speed.
The weight is adjusted simply by turning the limb bolts in or out. This changes the limb pad angle, and therefore the weight.
With the bolts tightened all the way down, the bow is at full weight. By turning the bolts out, you can adjust weight and tiller. It's about 1 lb. per full turn, with a max of 5 turns. A very cool thing.
Mark