What is SDI?

I heard that SDI doesn't exist in Smash 4

Okay. How should I SDI then?

Examples of practical applications

Help! Escaping <move> is really hard!

SDI, or Smash Directional Influence, is the act of inputting directions during hitlag (freeze frames when hit by an attack, before knockback occurs) to shuffle your character in that direction. This can be used to escape combos and multihit moves. In theory, it can also be used to position yourself to tech an attack or survive attacks that would barely kill you by moving further from the blast zone, but these scenarios are highly situational and typically unrealistic.Untrue! However, there are some factors that make it less powerful than in previous games:1. The distance of an individual SDI input was approximately halved compared to Brawl.2. The C-stick is no longer useful for SDI; Smash stick overrides control stick inputs entirely, while tilt stick doesn't affect SDI at all.3. Many multihit moves use autolink angles that are designed to hold you in the move.This combination of factors has led to SDI being an ignored and underutilized technique in this iteration of Smash, but just because it's nerfed doesn't mean it isn't still useful!Since the C-stick is unhelpful, SDI is optimally performed the same way as in Smash 64; by wiggling the control stick in a quarter circle, with the direction you want to go at the center (i.e. to go up, wiggle). Though performed on an N64 controller, this video demonstrates what you should be inputting for left > right > up > down SDI as the principle is the same:A good way to practice is with Smart Bombs. Go into training, spawn a bomb and hit it, then SDI up and try to escape before the final hit. The higher your percent is, the easier it is to escape; 50% is a good percent to practice at, but it's possible to escape lower than that if you want to challenge yourself.Multijabs:Yoshi dair:Mewtwo usmash:Witch Twist:Ryu utilt strings:Many multihits are structured in a way that requires you to understand the attack in order to escape it properly. For example, Pit's rapid jab is difficult - if not impossible - to escape by SDIing away from Pit, as it pulls you toward him. However, it does nothing to prevent you from SDIing up and jumping out. Mewtwo's usmash hitboxes form a diamond and the move aims to trap you in the center, but the attack isn't very good at pulling you down from the top.Unfortunately, if you aren't familiar with reading data dumps you're likely SOL on learning hitbox layouts for now (hitbox viewer when). Even then, some attacks you just aren't escaping (Sheik uair, Bayonetta jab combo, etc).