Here, I think Tod is demonstrating something about metal springs. I am an engineer, but not formally trained as a mechanical engineer. So, I do not know as much about metallurgy as I would like. However, I think the concept he is illustrating, as has also been discussed on MyArmoury, is that metal springs seem to have a limit to their velocity. Again, not as informed as I would like to be, but his test here seems to be leading up to this point. So, it will be interesting when he does a comparative analysis with a 1200lb crossbow – with a steel prod.
…The latter part is important. There might have been a trade-off for armies and soldiers for why they might want steel prods or composite prods. You might get faster velocities and thus longer ranges out of a composite prod, but you might have greater safety when using a steel prod. That greater safety is most certainly one of the reasons you do not see reproductions more like Bichler’s on the market, along with a heat treated piece of steel much more easily obtained these days as well.