She was using the Force to guide her movements for <10 seconds, long enough to clip Kylo's leg, which effectively ended the fight. Kylo Ren was caught off guard, briefly overwhelmed, and then injured.
This is a bit reductionist, though, and ignores context. In the film Rey used the Force to briefly overwhelm an injured, caught off guard, emotional mess of a man who was busy gloating over someone he thought he'd defeated. The moment is a clear parallel to ANH, where she takes the advice of a Force user to tap into it and rely on instinct, and let it guide her. (We'd already seen how powerful Rey was when she pushed back on his mind reading, earlier, which helps set this moment up.)