Back Slash is one of Shulk's physical arts in Xenoblade Chronicles, and it was made his side special move in Super Smash Bros. for Nintendo 3DS / Wii U and Super Smash Bros. Ultimate.
Upon using the move, Shulk leaps and slashes in front of him with the Monado, dealing moderate damage and knockback to whichever opponent he hits. As the name suggests, the move's damage and knockback increases significantly if he hits an opponent from behind. The move cannot be canceled once it is used (unless Shulk is hit during the attack), and does not end until Shulk hits the ground. Due to the short distance covered by the leap, Back Slash can be used as a recovery move, albeit limitedly.
The move has two custom variants in Super Smash Bros. for Nintendo 3DS and Super Smash Bros. for Wii U:
Back Slash Leap has Shulk leaping up, and then slashing down towards the ground. The move covers more vertical distance, at the expense of reducing its horizontal movement.
Back Slash Charge grants Shulk super armor while performing the slash, and offers more horizontal movement than normal. However, it deals a lot less damage and knockback than normal if it does not strike the opponent's back.