Small step forward smash
From SmashWiki, the Super Smash Bros. wiki
The small step forward smash is a technique in all four Smash games that effectively extends the range of a forward smash by intentionally delaying a few frames between the control stick tap and the attack button press. As the game allows a specific window of time between the two inputs (3 frames in Melee) in order to differentiate the combination from a dash attack, this causes the character to dash forwards slightly before the forward smash begins, therefore increasing its range based on how far the character moves in their early dashing animation. This can also be used to intentionally decrease the range of a forward smash for small positioning advantages.
In Melee, there is only 1 frame available to delay instead of 3 when doing a forward smash in the opposite direction, making it pointless. On the other hand, in Smash 4, the player can choose when to forward smash in the opposite direction.
The small step forward smash is most useful for characters whose forward smash has low startup and high range (such as Marth), effectively increasing the range slightly further. Pivoting can be used in a similar, yet more universal way, but it is much harder to do and also takes one frame longer.