This is commonly referred to as a “b-reverse” in Super Smash Bros. discussions. This is a technique that allows special-move direction to be reversed right after input, but before action begins. This is especially relevant when it comes to neutral specials. You might not be able to turn around at all in midair, and turning around on the ground might involve turning or pivoting time you don’t have. It’s invaluable being able to start a neutral special facing the wrong way, then snap it to the other direction. Almost all special moves can be special-reversed, but there are some exceptions (important to note to avoid disaster, if you try special-reversing something that can’t).
To perform a special reverse, input the special move and immediately follow with a control stick input toward your fighter’s back. The reversed-direction input must be extremely fast, like releasing Jump before takeoff to trigger a hop instead of a jump. Special-reversing is especially important in midair, where you might need to reorient a neutral special as mentioned, but where you also might have to start an up special aimed the wrong way, immediately correcting direction after input. This technique takes practice (slowing speed in Training Mode once again shows a place it’s useful for learning), and it’s important to learn which of a fighter’s special moves can be special-reversed, and where this might be valuable.