Shuttle Loop is the name of Meta Knight's Up B attack. Seen in this screenshot and in a few videos posted outside the Dojo, Meta Knight jumps into the air with his sword pointed upwards, goes into a loop and glides forward with his blade extended. The Shuttle Loop does around 9% of damage, and has good knockback.
The Shuttle Loop is actually a pre-existing maneuver. It was first used by Kirby in the SNES game Kirby Super Star when he received the Wing ability.