From the Super Mario Wiki
Dimensional Cape is Meta Knight's down special move in both Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS / Wii U. When used, Meta Knight disappears and reappears, facing the opposite direction. By holding down the button, Meta Knight attacks when he reappears, and tilting the Control Stick or Circle Pad causes Meta Knight to reappear in that direction. When used in the air, Meta Knight enters a helpless state.
In Super Smash Bros. for Nintendo 3DS / Wii U this move can be customized with two unlockable variants:
The first is Shield Piercer, which makes Meta Knight's attack unblockable, but its startup lag and ending lag are increased, and it deals much less damage and knockback than normal, making it more risky to use against low damaged opponents. It also does not travel as far as Dimensional Cape. Oddly, despite being unblockable, Bayonetta's Witch Time can counter Shield Piercer.
The second variant is Stealth Smasher, where Meta Knight travels farther than normal, and his attack deals more damage as well. However, despite its name, the move does telegraph where Meta Knight will strike, which makes it easier for opponents to detect Meta Knight's location, and even counter his attack.