Crouch Cancelling is one of the stronger techniques in SSBM. By cutting KB by 1/3rd, CCing a move can frequently allow you to counterattack from a grounded position before your opponent can follow up. With CCing, “Don’t Get Hit” is incomplete. In the right situation, getting hit can have a massive reward.
The inherent weakness of CCing is that it requires you to crouch. Crouching is conspicuous. This makes CCing somewhat telegraphed and easy to recognize and punish with a multi-hit move or grab. Crouching also severely limits your mobility. You might recognize that Dash Dance heavy characters CC with far less frequency than others. A Falcon player is less willing to surrender his capacity to dash back than a Sheik main.
But Melee offers us a compromise in the form of ASDI down.
ASDI down results in a sort of pseudo-CC. At %s that do not trigger tumble, landing will interrupt hitstun with a normal, 4f landing animation. ASDI will move your model a small amount downward. Additionally, ASDI triggers a ground collision. 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.
But the real beauty of ASDI lies in that it can be buffered and is not recognizable to the opponent before-hand. ASDI does not require the precision of an SDI input. You just need to be pressing a direction on either stick on the last frame of hitlag. It doesn’t matter if you pressed down before or during hitlag. This means that you can habitually press down during the startup and endlag of your moves and simply mash Z if you get hit out of them. Because you can buffer ASDI with the C stick, you can maintain your full mobility while preemptively buffering ASDI down just in case. This is very easy and non-intrusive, especially if you claw. (Theoretically you could even plug your controller in with the C stick pressed up lol).
Many players impulsively throw out a quick jab or tilt after making a lightly unsafe decision in an effort to beat out a potential whiff punish. This is highly effective but only to a point. Habitual ASDI will net you at least a grab off of these counterpokes, rendering them completely fraudulent.
Once you start looking you will see this technique with very high frequency in high-level matches. I would go so far as to say it is one of the biggest mechanical differences between top and mid-level players.