Kevlar works due it nature of being extremely difficult to tear. Unfortunately it is also easy to cut. This is why (in concert with its light weight) kevlar is ideal for bullet proof vests. I would propose that the fencer's blade had a "relatively" sharp edge/point where it happened to make contact with his opponent's kevlar protection, therefore going through the fabric with relative ease...

> How it stops thing may have something to do with performance
> in specific applications. For instance, chaps worn by lumberjacks
> keep a chainsaw from cutting your leg by the kevlar turning into "cotton
> candy". I'm not sure whether bullets are deflected or absorbed
> by a kevlar vest.