Drop canceling, also known as platform canceling is a tech introduced in Super Smash Bros.. It is essentially the fastest way to use an aerial attack, sometimes leaving only one frame before the aerial attack's animation starts. In comparison, DJCFFL has 5 or 6 frames (usually as a result of human error or jumpsquat) before the attack initiates.
In 64 and Melee, while dropping down from a platform - one frame after the initial crouch animation - characters are considered to be in the air. Because of this, they are obviously allowed to use aerial attacks. Drop canceling takes its name from the fact that using an aerial attack during that one frame causes the character to re-land on the platform, thus canceling the drop. It's important to note that it is far simpler to drop cancel an aerial attack that shifts the character's body upwards (like Falco's back aerial or Captain Falcon's up aerial). Conversely, it is harder (and possibly impossible) to drop cancel aerial attacks that generally shift the character downwards (like Marth's down aerial).
In Melee onwards, there are many deciding factors to whether an attack can be drop/platform canceled. How strong the attack is, and thus how much hitlag it has, how fast the aerial's hitbox comes out, the ECB pattern of the aerial and the character's fall speed all affect an aerial's ability to drop/platform cancel.
After Melee, Drop Cancelling changed slightly. Although traditional drop canceling does not return in later games, certain stall-then-fall aerials can emulate the effect, albeit with varying utility and effects. For starters, the startup lag isn't always shifted to one frame before, and an L-Cancel isn't necessary to perform. Overall, while it becomes slightly easier, the payoff is nerfed a smidgen. However, characters like PAC-MAN profit heavily off of this due to the combos it can enable.
How to perform
This happens because the character's ECB changes position after a certain number of frames after the aerial. Due to the additional frames of hitlag on shield, some moves will progress to this state before dropping too low.
Applicable attacks in Melee
Unless noted, attacks have a one frame window. These apply to stationary platforms; for downward moving platforms, the window sizes may vary, and more attacks may be possible.
Pseudo Platform Cancel (Melee)
When sliding off a platform you start falling with lower velocity than when dropping through a platform. This allows you to cancel many more aerials than a regular platform cancel because you effectively have more airtime. Here are 2 examples of this phenomenon.
In these examples, Marth and Falcon use aerials which cannot normally be platform canceled. They are hit in shield and shield DI towards the edge, which pushes them off the platform, then the knockback pushes them back on the platform.
These still require hitlag though, as hitlag allows the ECB to update (and thus become above the platform). Samus, however, has such low fall speed, that she can throw out a nair, have her ECB update, and be above the platform, without any use of hitlag.
Applicable attacks in Brawl
Applicable attacks in Smash 4
Applicable attacks in Ultimate
Largely the same as in Brawl and Smash 4 in execution.
When used with aerial attacks that have little lag, a drop-canceled aerial attack can lead to lethal combos due to the incredibly small amount of lag. In comparison to the SHFFL, the character's jump animation and falling animation are both eliminated, removing somewhere from 20-30 frames from an aerial attack. However, drop canceling could easily be considered the most difficult to use advanced technique in the game, requiring frame-perfect technical skill to be used consistently and effectively.
It is also possible to do this in Smash 64, as demonstrated through TAS style combos by Jpheal, and it is similarly incredibly difficult. However, it is only possible on rising platforms/diagonal platforms.
Similar to drop canceling is the aerial interrupt: a technique in Melee that causes a character to land on a platform earlier by doing an aerial attack at a certain frame. The reason it works is that the character's environment collision box (ECB) is shifted downwards due to the attack's animation, making it connect with a platform and triggering the landing animation. It can be used to preserve enough ledge intangibility so that a grounded attack can be carried out whilst being still intangible. Its use can replace the need to air-dodging onto a platform since the aerial interrupt is the fastest possible way to get onto a platform from under it.