I have no idea why this topic is being discussed in a Rogue One thread… but here’s my two cents:
It seems that many people always forget the Maz / Rey moment when they bring up the lightsaber fighting.
Close your eyes. Feel it. The light… it’s always been there. It will guide you.
So my understanding has always been that it’s the Force guiding her saber when she truly gives in / concentrates and goes full “limit break” on Ren.