Easy way to do this trick is to bind Forward and Back twice on ur pad on different keys ( if u play with a PS2 Pad for example).
So HP , LP , HK , LK are on the 4 actionbuttons and on L1 = Block R1=Run L2=Back and R2=forward
If u hold up on diagonal buttons and keep tappin/buffering with L2 and press HP u will get the air fireballs instant low to ground , from there its finding the pattern/rythm to get it of consistently.
Having Forward and Back on 4 buttons instead of 2 makes some other moves alot easier/less annoying to do.
Like Kabal's Spin , Rep's Dash , NW 's Axes , Jade's invis.
u will get them out quicker and much more consistent. And suddenly kabal and rep turned into my fav chars. No Macro or Turbo needed , and keys can be binded like this in Mame , and in Epsxe u will need to unbdind R2/L2 within Epsxe and use xpadder to bind these like this.
I hope it helps.