User blog comment:Varzan Deodinm Beldinm/Volume 2: Will the Fan Feedback Pay Off?/@comment-72.94.58.201-20140702181259/@comment-2006510-20140702224705

In regards to the Ursa situation, Pyrrha only manipulated Jaune's shield - she did nothing about the sword, as her Aura was only seen on the shield. So despite wear-and-tear from 4 generations of presumed use, Jaune still took off that Grimm's head with a single stroke of his grandfather's sword

It's not unreasonable that it could have gone the other way without her help, due to simple luck. He needs more training, certainly, but still.