Basically, if you grab the ledge after a B special that leaves you in a fall state, it cancels the landing lag. However, this landing lag gets "passed on" and the next time you are airborne, you will recieve this landing lag when you hit the grou
This doesn't work with all characters though. The characters affected are Ganon, Wolf, Mario, Luigi, Fox, Falco, Marth, Capt. Falcon, Charizard, Squirtle, Sonic, and Lucario.
As ana example, say you UpB to the ledge with any above character. You then ledgehop and land behind them and shield. Because of this landing lag glitch, you lag more than a normal jump, and your shield won't come out as fast, opening you up for an attack.
Ready, steady, deadly...