Directional influence

Directional influence, abbreviated DI, is the control that an attacked opponent has over his or her trajectory. It originated in the  series and can be performed in Super Smash Flash 2.

When an attack sends a character flying, directional influence can be used to alter the trajectory of the target. DI cannot reduce or negate the momentum of a move's knockback; attempting to do so will only make the trajectory straighter, harming the player's chances of recovering back to the stage.

When knocked vertically, pressing the left or right key will slant the trajectory of flight sideways, causing the character to fly diagonally. Similarly, pressing up or down when knocked horizontally will slant the trajectory of flight upwards or downwards, again towards a diagonal flight path. As the corners of a stage are further away than the sides of the stage, the momentum that would kill a character when sent flying straight up or straight sideways is survivable if directional influence is used to make the character fly towards the corners of the stage.

Smash directional influence
Smash directional influence, abbreviated SDI, is a mechanic that allows players to slightly alter their position during the freeze frames of being hit by an attack; before getting launched. To Smash DI, the player presses any direction of movement or the C-stick during the freeze frames to move the character slightly in which ever direction was input. SDI can be used to move into a wall or ceiling to tech, break out of combos, or move away from the blast line.

Trivia

 * Oddly, both SDI and DI are referred to as simply "sdiDistance" in the code.