This is a
Street Fighter series combo. Well, a mini
combo if you will. The object of it is to get two hits in the space you'd normally reserve for one hit. The
technique includes just double tapping the button you want to use in the middle of the controller motion for the special move you want to use (it only works with certain moves). The
fireball two-in-one requires that you tap
fierce right after the downward pull of the
joystick, and then complete the fireball motion and hit fierce again.
In standard SF notation:
D,HP,DF,F,HP
It takes a little while to master, but it's worth it, especially when you consider the utility you can get out of it.