Drop-counter
From SmashWiki, the Super Smash Bros. wiki
Drop-counter is a technique in all three Super Smash Bros. games. To do it, you must be at the edge of a platform (not hanging, but standing right at the edge). When your opponent attacks with a short-ranged attack, shield against it; if the knockback is strong enough, you might slide off the edge of the platform, canceling your shield and allowing you to retaliate instantly with an aerial attack.
This technique was also known as the Marth killer in SSBM, as when Marth would use his Up-B 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 was to sweet spot the ledge, or recover on the stage completely.
