A reversal backstep is a backstep used as a reversal, particularly on wakeup (i.e., getting up from knockdown).
This technique is intended mostly to take advantage of the midair hitbox that accompanies a backstep in certain games (or with certain characters), allowing one to evade the opponent’s command throw attempt against one’s wakeup.
In games that give a backstep an invincible hitbox, it can also evade strikes, which is particularly for characters who lack invincible moves. However, this does incur risk; if the opponent reads this ahead of time, they will simply attack aiming for when and where you land.