The easy input method is typically pressing forward and pressing A at the same time and then released A. This is better expressed as these steps:
The advanced, negative edge, input method takes advantage of the fact that most moves are triggered by the release of the button. Thus you can typically do:
, shaving milliseconds or much more of your input time. For example you can use this with Cable in MvC2 to throw grenades that detonate right next to Cable's head, as opposed to their usual usage as distance weapons.