The working mechanism for this crossbow is in the arms made of rolled paper and supported by tape. Typically a bent tube of rolled paper would tear when bent, but a layer of tape seems to distribute the load evenly enough to not only prevent the paper from tearing, but also give it a strong and fast spring action when released. This allows for a rigid bow string to be used which utilizes the energy in the arms to propel the projectile - Rather than most toy crossbows which use rigid arms and an elastic band which propels the bolt more like a slingshot than a crossbow.
This crossbow can throw a pencil or pen a good 100 feet. A possible modification for even more power might be to make two arms on each side that the bow string would tie together at the tips. Having double arms is about the only way to go about adding more power because adding much more paper to the first set will only result in them tearing or being completely rigid. Similar to how you can only fold a piece of paper so many times.