Interruptibility

IASA frames are just what they sound like: The frames at which you can first interrupt an attack.

It's most noticable as Marth. Do a Downtilt and watch the entire animation. Now do a Downtilt and follow it up with any other move or a dash and notice how much shorter the lag of the Downtilt became.

Peach can utilize the IASA frames to perform the cool, albeit impractical and pretty useless Vidjo-cancel and Vidjo-drop.