Vision Sliding is an advanced technique in Smash 4 which allows Shulk to significantly amplify the distance he moves upon the activation of Vision's counterattack, along with the ability to have complete control of the counterattack's momentum.
Vision Sliding has been triggered inadvertently by players since the game was released, but it wasn't until September 2016 that Shulk mains Erico and Paradigm fully researched it. Explanations of the phenomenon along with exact setups for buffering Vision Sliding on specific stages can be seen in their video guide.
Vision Sliding can only be performed in Smash 4; the bug that allowed this technique has been fixed in Ultimate.
How to perform
Vision Sliding only occurs when Shulk activates Vision on the exact frame he transitions from air to ground. As the timing for Vision Sliding is very strict, buffering Vision out of aerials is commonly done to make this easier. The standard example is using a full-hop down aerial buffered from Shulk's jumpsquat, then buffering Vision during the move to force Vision to activate immediately after landing. So long as no Monado Arts are active (or any Art that doesn't change Shulk's jump height), this will result in Vision Sliding if an attack is countered.
Like a normal grounded Vision, Shulk can trigger both a standard and forward-held counterattack. However, Vision Sliding grants Shulk the ability to change the direction of the counterattack's slide by holding a direction, compared to the standard version which has no ability to do so. This freedom of movement even makes it possible to slide forwards and then backwards part-way through the animation by inputting the respective directions on the control stick.
Once the initial perfect-landing Vision is inputted, all subsequent Vision counterattacks used in a row will Vision Slide until the player stops using the move. Additionally, Shulk is able to slide a tiny distance forward or back during Vision's pose, and it can be chained repeatedly just like the counterattack itself.
The distance of the slide is dependent on the air speed and air acceleration granted by an Art. The Monado Arts with the longest to shortest slides are follows: Jump, Speed, No Arts/Buster/Smash, Shield.
The primary applications of this technique include punishing projectiles that would otherwise outrange Vision's hitbox (similar to one of Shulk's custom down specials, Dash Vision), as well as baiting opponents to attack Shulk due to his high landing lag, as special moves can be used before landing lag occurs as a way to defend himself. In teams scenarios, Vision Sliding can also be triggered by a teammate, allowing its large range to hit multiple opponents (even if shielding).