Shulk performs a small leap, announces "Back Slash!", and does a wide downwards slash attack with the Monado. The amount of knockback and damage inflicted varies depending on the direction the opponent is facing; a successful attack on an opponent's back will deal bonus damage and put them into a different knockback animation, facing away from Shulk, the same animation triggered by bouncing off a wall after suffering knockback. Back Slash's damage ranges from 9-10% with negligible knockback from the front and 16% with considerable knockback from the back. A successful back-hitting Back Slash becomes one of Shulk's most damaging attacks in Buster mode, dealing 22% with massive shield damage, and is also one of Shulk's reliable KO moves with the Smash art, KOing at around 90% at the back (middle of stage) and 118% at the front (near the edge). Aesthetically, a front-hitting Back Slash makes a typical slashing sound, while a back-hitting one makes a deeper, stronger slash sound, the same sound used when Ike's Ragnell attacks connect.
The small jump that Shulk performs when using the move can jump over small projectiles, and the slash can catch opponents off guard with its wide range, giving it some use as a surprise approach option. It is useful for punishing laggy attacks, specifically those that cause the opponent to end with their back facing Shulk, for tech or roll chases if the player reads an opponent's dodge, and can be used after a back throw to hit an opponent who attempts to jump back after being thrown. However, the attack has high ending lag, making it unsafe if shielded. In Ultimate, the jump distance can be increased or decreased by inputting right or left during the jump respectively.
Shulk cannot cancel out of the move when he has initiated it: as a result, careless use of it can result in opponents punishing the attack's lag, and it can also result in a self-destruct if used too close to a ledge. In addition, despite the small lateral movement Shulk gets from initiating the move in the air, utilizing it as recovery is risky and ineffective as the move does not sweetspot ledges. In version 1.1.0 of both games, Back Slash's startup was reduced by 9 frames, making it moderately easier to land.
It is also one of the few moves in Smash 4 with a back-hitting hitstun animation, as the opponent does not automatically turn around when receiving a hit from this attack.
Something to note is that Back Slash's unique property works on more than just playable characters. It also behaves correctly on Smash Run enemies, Sandbag, Master and Crazy Hand, stage bosses, entities summoned from an Assist Trophy, Pac-Man's Fire Hydrant (which matches which direction Pac was facing when he deployed it), and even Smash Balls, among other things.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Back Slash is one of Shulk's physical Arts in Xenoblade Chronicles, and is one of the two Arts (excluding his Talent Art) in his arsenal when he is introduced, being his main physical art. It does the most damage out of all the physical arts in his complete moveset, and as a bonus effect, it deals double damage when hitting the enemy from behind. Its low cooldown makes it one of the most potent Arts Shulk has, though this is hampered by needing to be behind the opponent for maximum effect, meaning other party members have to draw the aggro away from Shulk (such as Reyn, whose main skill increases enemy aggro towards himself). Otherwise, this attack deals standard damage if hit anywhere else, although the damage is still higher than an auto-attack. The difficulty of landing the move in Smash mirrors this behavior, as opponents need to be caught offguard for a Back Slash to land and rewards Shulk for hitting backs with either increased damage (Buster) or a reliable KO move (Smash). The red color of the Monado's beam is a reference to the red color which identifies physical Arts in menus. Other characters in Xenoblade also have Arts with the same bonus effect as Back Slash, examples being Fiora's Power Smash and Riki's Sneaky.