It's SDI.
When you are hit by a move, both you and your attacker suffer freeze frames. During these freeze frames, you can move just a little bit by holding the control stick. This tiny bit of movement can prevent subsequent hits on multi-hit attacks (such as Fox's uair).
ASDI is the slight shift in direction once freeze frames end. It's essentially a weaker, slower variant of SDI. Fox's uair is pretty fast, and ASDI won't save you if the second hit is lined up properly, whereas with SDI you have a chance to live.
Take this for an example.
If you DI Fox's uthrow to the right, he has to move right to hit you. If he connects the first hit of the uair, if you SDI left, the second hit will have less of a chance to hit you because of his rightward momentum. However, if you SDI right, you're practically SDIing along with his rightward momentum right into the second hit. SDIing upwards can sometimes work if you're floaty enough.