From the Super Mario Wiki
The Upperdash Arm is an Arm weapon from Kid Icarus: Uprising that appears as Pit's side special attack in Super Smash Bros. for Nintendo 3DS / Wii U, replacing Angel Ring from Super Smash Bros. Brawl. When used, Pit dashes forward and uppercuts any opponents in his path. The move can also deflect projectiles when executing the uppercut.
This move has two variants: Interception Arm and Quickdash Arm. Interception Arm turns the move into a counter move, allowing Pit to counterattack opponents with an uppercut when attacked. Quickdash Arm has a longer and faster dash which continues after dealing the attack.
Super Smash Bros. for Nintendo 3DS Trophy information