Its a tricky design to produce, thats for sure. You want the little bow bend early in the draw, and then as its string pulls tight to the back what you find is the big bow limbs get stressed more at mid-limb than it would without the little bow. So you're drawing two bows, and changing the stress on the big bow all at the same time. The hard part is figuring out what dimensions to make the little bow. You want some reflex because that will give you some amount of bending wood, and the relative length of little bow to big bow is going to change how soon you get to "full draw" on the little bow, at which point it sort of becomes a cable-type backing for the big bow. Plus you want the little bow to be fairly stiff, so it doesn't "max out" too easily. YOu want it to be pulling against you for most of your draw. Then you have to properly tiller the big bow so it has a decent bend, which I'm told comes down to making it stiff at mid-limb before you add the little bow.
I've never tried one, I personally think they look kind of clunky. But they do have interesting physics.
