Imagine an Xbow with 40 pound limbs. My thinking. You were going to pull 40 lb with a rigid fork which itself feels the same 40 pounds but it is fixed. If you pull 40 pounds with flexible limbs, they feel the 40 pounds and they bend along with the rubber being stretched. When you release the rubber it is relaxing along with the limbs returning to their unflexed position. Will the extension of the rubber be the same in both cases? Certainly, if you could cock the limbs to 40 pounds, then draw the rubber to 40 pounds, you would have 80 pounds of potential energy. Can you think of a way of triggering both?