Marth Tech: Smash DI
Description / How To Perform
During the pre-knockback freeze-frames of an attack your position can be slightly affected with inputs on either the Analog or C-Stick. During each frame, the game will accept a single input from each stick. If a direction is detected, your character will be moved slightly in that direction.
Holding the analog stick in a single orthogonal direction for multiple frames will not result in the game interpreting multiple inputs for SDI. However, the C-Stick does count each frame as a seperate input (see 
ASDI).
This means that to register multiple SDI inputs with the analog stick you must repeatedly tap a direction quickly, or quickly alternate between two adjacent controller notches (sometimes called Quarter-Circle DI).
Difficulty: Dependant on Skill Level
Usefulness: 
(Extremely Useful)
Extra Info
SDI is difficult to implement in most regular combos but is terrific at countering some common attacks, or at least making them harder to followup on. For example, the second hit of Fox's Upair can be avoided by using SDI to move far enough away during the first hit. Also, you can avoid giving him an easy Drill into Shine by SDI'ing through his Drill to outspace the shine. This actually isn't as hard as it may sound, since you can SDI on each of the individual hits of the Drill.
SDI can be used to cross through Captain Falcon's Gentlemans Combo and Nair in certain circumstances to save you from followups.