It's explained in the translation spreadsheet. Basically unlock and equip magic cancel _______ -RA/-GA. _______ being whatever move magic cancel is branching from (for example Sice' forward/backward directional attack) then you perform that move and as it finishes, press or hold the magic attack button and it'll fire out. It's really only makes a difference when you unlock -RA/-GA MC --- Official ALEXANDER of the Final Fantasy Type-0 Borard _\(=/)/_???
It's like, "FEEL MAH KICK!" and then "Opps, I dropped DAT BOM2" --- "You won't kill me out of some misplaced sense of self-righteousness. And I won't kill you because you're just too much fun. I think you lack hatred"