“ In game, if you are hit with a low-mid KB move at low %s and then ASDI down then your character will go straight from hitlag to 4f of landing lag, leaving you with frame advantage comparable to CCing. This will work to great effect up until the trajectory of f1 after hitlag exceeds the ASDI unit of distance at mid-high %s. ”
— Alex's Puff Stuff
ASDI down is one of the most recent developments in the defensive game of Melee. While ASDI itself has been understood for a while, widespread abuse of floorhugging and taking attacks to get a stronger punish is more popular than ever, to such an extent that “holding down” has almost become a meme.
ASDI down does lose to set knockback moves and grabs, but it has massive offensive and defensive utility. It can be used to Tech moves like Sheik's dash attack instead of getting launched by them; it can wall-break characters like Falcon that rely on retreating back-air; and on platforms, it can set-up slide-off reversals.