Jaune decides that he's done holding back his power and is now using his full speed. Basically Jaune got struck by lightning and his family became overprotective, wanting to calm down he went for a run and accidentally went to the smallville universe and became Bart Allen when he was 14. After an accident and the death of his friends he returned to remnant when he was 16 and decided to become a huntsmen, but not use his speed. Flash/Impulse is owned by DC and Warner Brothers and RWBY is owned by roosterteeth.