ASDI is just like SDI which directly repositions your character slightly in that direction except the amount of movement is smaller, and instead of happening duringhitlag it happens automatically (it's what the 'A' stands for) on the 1st frame of knockback right after hitlag based on the direction being held on either the control stick or c-stick, so unlike SDI you don't need to actually smash the direction for it to happen. Since it happens by simply holding a direction on either stick as hitlag ends you are ASDIing stuff all the time except it isn't usually noticeable since it's both a small distance and is also combined with the KB itself.

If a direction is being held on both sticks, then the direction on the c-stick is what's used for the ASDI. Because of that part, it lets you directional DI to change the launch angle on the control stick while also ASDIing in a different direction than the one being held on the control stick.

The reason holding down on the c-stick does a weaker 'crouch cancel' is because it does the main component of it which is ASDI downwards. A normal CC by holding down on the control stick is a combination of crouching (reduces the launch power/KB and halves the hitlag you get from the attack), ASDIing down into the floor to instantly land on the ground on the 1st frame of KB, and usually holding down also causes a lower launch angle which reduces the amount of upward movement from the KB that the ASDI has to overpower to keep you from leaving the ground.

ASDI like SDI is mostly helpful for escaping multihit attacks, low knockback hits/combos where normal trajectory DI has less of an overall impact on where you end up after the KB, for certain things like wall teching, and stuff that's specific to ASDI are for CCing and ground teching (since floors can't be SDI'd into).

You can also A/SDI during shield hitlag (c-stick can't be used for it obviously since it'd make you roll/dodge/jump with the shield being up). Shield ASDI is actually what makes the lightshield edgehog on Marth work. You ASDI off the edge and when the movement offstage from the ASDI is greater than the shield pushback from the attack towards the stage you slip off the edge and fall.

Holding towards someone while shielding their attack will make you ASDI towards them which keeps you closer and allows you to shieldgrab some things that you'd normally be pushed too far away to reach them. Doing the opposite, you can hold away while shielding Peach's d-smash to take less hits and damage to your shield while getting pushed out of range sooner to be able to punish.

Another thing you can do with shield A/SDI is stuff like SDIing Falco's shine away in your shield into a grab so if they multishine it doesn't reach you and they get grabbed, and if they jump out of the 1st shine then Ganon's tall enough to grab them out of their jump.

Using the c-stick to ASDI is only useful when the direction you want to ASDI is different from the direction you're going to be holding on the control stick. The reason being is that ASDI, which is the only thing the c-stick does as far as DI, is also read from the control stick anyway. Holding both sticks in the same direction results in the same thing as holding just the control stick in that direction.

ASDI happens on the same frame as the 1st frame of KB, while SDI happens before KB, so technically you're moving towards the ground while also moving up and away from the f-smash's KB at the same time. Whatever the net distance of those 2 is is your movement on the 1st KB frame. If the total vertical movement is into the ground you'll land in your regular air->ground landing animation if the KB is weak enough to not cause a knockdown/tumble (2, 4, 5, or 6 frames depending on the character), or you'll fall down directly on the floor or tech if the KB is strong enough to knockdown.

Anything that hits on the ground and has a launch angle of horizontal or lower and the KB is weak enough to not cause a knockdown can't be trajectory DI'd or A/SDI'd upwards or downwards (grounded stomps still launch downwards, they just bounce you off the floor and you go upwards once it begins causing a knockdown). Some examples are Fox's shine, Sheik's needles, Falco's d-air at lower damages, and the weakest hit of Ganon's reverse u-air.

Since the 'cancel' part in 'crouch cancel' requires you to ASDI down into the floor to cause a regular landing to cancel the stun with that animation, these attacks can't be crouch canceled. They can still be crouched, but all that does is reduce the launch power which reduces the stun by some since stun is calculated from it.

He's in hitlag that entire time, so there's no KB actually happening at that point. SDI is what allows him to move around like that.

When rotating the stick along the outside for SDI it triggers by passing specific points depending on which direction you're rotating.

The ones with Pink arrows are the trigger points for clockwise rotation and the Blue arrows counter-clockwise. Once you cross over/pass those points it will trigger a SDI in the direction your stick is pointing if you're in hitlag. The Orange area is the area that can trigger SDI. Having the stick within the inner Blue area can't cause SDI.

Since there are only 4 points for a direction of rotation, some rotations are inefficient for getting a SDI. Starting at Down-Right and ending at straight Up would only cross one of those points and would result in 1 SDI, and so would starting at full Right and rotating like 10° upwards. If you were to start Right and go up slightly to cross that counter-clockwise point, and then go down slightly down to pass the clockwise point right below it you'd get 2 SDIs from rotation from only about 30° of total rotation.

It doesn't work for most characters when the hitlag is 10+ frames (hitbox does 21%+ of damage for normal attacks, or 12%+ damage for electrical attacks) unless you SDI back down towards the floor since they get lifted up and are no longer touching the floor after 9 frames of being in hitlag. Only DK, G&W, Jiggs, Kirby, and Mewtwo stay along the floor after that point. If the hitlag is 9 or less than every character can do it just fine.

Also, it's only really useful for surviving KO hits with launch angles that are about 50° or lower. When the angle is too vertical the upwards KB starts overpowering what the ASDI down can do before the point you'd start dying from the attack anyway.

Characters with extremely low traction and poor recoveries from low and out like Luigi will slide very far off afterwards and be unable to make it back much earlier than if they just survival DI'd it. It's probably just Luigi who's better off always DIing normally and not teching for diagonal and lower KO hits though.

also this includes a list of attack's launch angles and the top speeds of those angles that are techable with dual-stick DI: