Edge slipping
Edge slipping is a mechanic that occurs in Super Smash Bros. Brawl where a character is knocked off a platform while they were still on their feet. This often occurs when a shielding character slides off a platform from shield knockback, but it may also occur when hitboxes that produce too low of knockback to lift a character off the ground are landed, which causes a character to slide off the platform.
Mechanics
What direction a character is facing determines the exact mechanics. If a character is facing forward while they slip off an edge, they are free to perform any action immediately upon falling off the platform. However, if a character edge slips in the reverse direction they were looking, they will enter their edge slip animation, and be unable to perform any action for a certain amount of frames. For every character, they are able to DI the fall left or right after the eighth frame, but are still unable to affect the vertical momentum of their fall or perform any other action. For how many frames a character will be unable to perform an action varies for each character; for example, Snake and Marth will be unable to perform any action for 27 frames. Once these frames are up, a character may jump or attack out of their edge slip animation. An important thing to note is that while in these frames during their edge slip animation, a character cannot tech upon hitting the ground, thus they will always end up in their knocked down state if they hit the ground during these frames.
How far a character slips off an edge is directly related to the knockback of the hitbox that pushed them off the ledge. For example, Luigi's forward tilt knocking a shielding opponent into their edge slip animation off a platform will result in the opponent moving barely away from the platform. But his forward smash would send the same opponent a considerable distance away from the platform.
Drop-counter
A drop-counter is a technique that can be performed in all three Super Smash Bros. games. The technique is performed by having a player standing next to an edge. Afterwards, a recovering opponent must use a short-ranged attack. The standing player must shield against it. The knockback must be strong enough to make the shielding player slide off the edge of the platform, allowing the player to retaliate instantly with an aerial attack.
This technique was also known as the Marth killer in Melee, as when Marth would use his Dolphin Slash to attempt to recover on to the stage, his sword would hit the farthest edge of the foe's shield, causing them to topple backward and edgehog the ledge, sending Marth to his doom. Two ways to counter this are to sweet spot the ledge or land on the stage.
Uses
Edge slipping is often exploited to set up the landing of a powerful, but normally difficult to land attack, to set up locks that work on knocked down opponents, or to simply set up a free punish. For example, while fighting on Battlefield, Player A is using Bowser while their opponent Player B is using Wario. At one point, Player A catches player B shielding on one of the middle platforms, from where Player A attacks Player B from below with an autocancelled short hop forward aerial. This causes Player B's Wario to slide off the platform backward and enter their edge slip animation, from where Player A immediately follows up with Bowser's forward smash, which hits Player B's Wario who cannot avoid the forward smash, resulting in a KO.
Another scenario on Battlefield involves Player C who is using Falco, and Player D who is using Donkey Kong. During the match, Player D is at 0% while standing on a middle platform with their back to the edge. Player C catches this and attacks with Falco's back aerial, which causes Player D's Donkey Kong to slide off the edge and into their edge slip animation. Upon Player D's Donkey Kong hitting the ground, Player C immediately follows up with Falco's Blaster, trapping Player D's Donkey Kong in a laser lock.
