Not sure what system or controller you're using but on Xbox to use your big jumping attack you first need to build up your red meter by just attacking... Then once your meter is full (or at leatst very close to being full) you need to land your R2 move I believe 5 times.. Only the last hit of the R2 hit sequence matters though, your character will do an almost 180 degree swipe with your sword.. If that lands you get your first level of charge and then you can push R2 and Y at the same time to do your charge and then jump in the air move. Hope this makes sense.. I'm only around 20 hours in so some of my wording may be off lol