Actually, thinking about it more, I would bet Kevlar works because it is highly resilient. I know, more boring terms, but resilience is the ability for something absorb energy when it is deformed elastically. It is the ratio of yield strength squared to young's modulus. Not going to get into that, BUT, the yield strengh of Kevlar is extremely high, and if it's a squared term, that could negate the high Young's modulus if it's a lower order. This is pure speculation on my part.