Directional influence, abbreviated DI, is the control the receiver of an attack has over his or her trajectory. Each attack sends its target in a particular direction, depending on the attack itself and on the target's weight and falling speed; DI can be used to alter, but not completely negate, this trajectory. This change, however, can be vital to surviving high-power attacks such as Fox's up smash, and for escaping combos such as Jigglypuff's space animal slayer, among many others. DI is most useful to make the character move into a trajectory that is as far from the blast line as possible. "Good DI" is when a character is sent in a trajectory that creates the greatest distance between the character and the blast line. In most situations, angling towards the upper-left or upper-right corners provides the best DI near the center of the stage, but the ideal angle of DI varies depending on the character's position on the screen. Conversely, "poor DI" (sometimes known as "PatG DI" after a smasher with particularly poor DI) is when the angle of trajectory creates the shortest distance between the character and the blast line, or the angle sends the player into a position they cannot recover from when they would normally easily recover from the blow (such as down and away from the stage). Bad DI that results in a death commonly occurs near the sides of the stage, when a character is hit diagonally while holding the Control Stick downward (commonly as a result of intentional fastfalling), which sends them on a more horizontal trajectory, towards the closest blast line.
In Brawl, the following are the most common ways of utilizing directional influence for surviving KO attacks:
The player can DI by pressing the control stick in any direction during or just after being hit by an attack. There are three types of directional influence: normal DI (often simply referred to as "DI"), Smash DI, and Automatic Smash DI. The type of DI changes depending on when the player presses the control stick.
DI in the Super Smash Bros. series
In Brawl, DI is not as useful as it was in Super Smash Bros. Melee, but it is easier to perform due to longer hitlag. Momentum canceling, a technique unique to the former, is of very high utility, and this coupled with DI enables players to survive high-knockback attacks at relatively high damage percentages for characters who can utilize it.
DI in Super Smash Bros. Melee
Survival DI alters the knockback direction towards the upper corners of the screen so that blast zones are escaped, preventing a kill if the amount of knockback wasn't too high.
Because of the high amount of hitstun in Melee compared to Brawl (or, more accurately, the lack of the ability to negate hitstun), it is often possible to hit the opponent with another attack while they are still in hitstun. By using DI away from the attacker, the victim can increase the distance the attacker needs to travel and thus make it harder for the attacker to reach them in time.
Most of the time, combo DI is as simple as holding left or right in the knockback direction. Against characters like Captain Falcon who have aerials with high knockback and low ending lag, it is often hard to decide between survival DI and combo DI. While combo DI might escape a consecutive hit, it might also be enough to pass the horizontal blast zones and die. Survival DI ensures that doesn't happen, but it will make it easy for Captain Falcon to land another aerial.
To determine the effective trajectory, the position of the control stick is read on the last frame of hitlag. ASDI is usually triggered by this as well. The highest deviation of trajectory is produced by directions perpendicular to the original knockback angle of the hitbox and amounts to approximately 18°. Holding directions that are parallel to the original angle will produce no trajectory DI at all. Because the possible DI angles are not distributed equally (see diagram to the right), knockback angles within 17° of vertical or horizontal can be trajectory DI'd to a lesser degree, as no true correspondent perpendicular angle is available.
Directional influence acts in a very different way in Super Smash Bros. 4. Instead of altering the angle of knockback by a limited amount, the player inputs a vector that is added to the vector of knockback. As a result, instead of surviving longer by DI'ing perpendicular to an attack, players survive longer by DI'ing directly against the attack, and can also escape combos by DI'ing in sync with the attack (effectively adding to or subtracting from the knockback instead of reorienting it).
In an attempt to distinguish the new mechanics from the old, some players prefer to call the new form of directional influence "vectoring", or "knockback influence". Another term gaining interest is VI (Vector Influence).