Some bows simply have a rising bolt-track, particularly at the very end of the power-stroke. If you do that right, it should give your bolts a little loft, resulting in a slightly longer point-blank range, and maybe that was the whole purpose back in 1500, but I have another theory: Perhaps the ramp is intended in extend the useful life of laminated horn/sinew composite prods. I figure any bow that you have to glue together will eventually come apart, due to a combination of flexing and shock of un-tapped energy absorbed at the end of the power-stroke. So if the bolt-track ramps upward at the end of the string-run, you Should get a slightly softer stop, as the friction increases, with less recoil and noise. Of course that means you're wearing the center-serving a bit more and twisting the prod a bit, but it should soften the shock as the string halts the prod, and that might be critical in a prod glued up with fish-glue.
Of course if the ramping starts too soon, you lose a bit of energy at the end of the power stroke, but I suspect most crossbows and handbows for that matter, don't get much push from the last 20 percent of the power stroke anyhow. Maybe it was worthwhile to sacrifice that last bit of push in favor of longer life for a Very Expensive horn/sinew composite prod. So does this have application to modern laminated bows as well?
Just a thought... comments? Geezer