Board Thread:Boruto: Naruto the Movie/@comment-26010916-20150806143152/@comment-3568954-20150811165240

I believe that she has been secretly practicing it i don't believe that was her first use of it.

notice how she states LOCK ON this implies it was a technique she had used before maybe she got a hold of some gentle fist scroll after awakening it

mind you there is no proof that this was her first time ever using the byakugan