If you know the visualization for the standard vibration of a rune, simply reverse the path. HPS Maxine gave the example of Fehu where one starts at the base chakra with FFF, proceeds to the left/feminine shoulder chakra with AY, and then HUUU for the left/feminine temple chakra. If you want to reverse it, simply vibrate uheF and start at the temple chakra (UUUH), go to the shoulder (AY), then the base (FFF).
One example is using the backward vibration of Gebo (Obeg, or, for Icelandic, föjG) to break up a relationship. Add the forward vibration of Yera first for extra power. This is one example of how forward and backward vibrations can successfully be used together in one working and work together.
If you have used the forward vibration of a rune for black magick - like Thurisaz - you can vibrate it backwards to help neutralize any negative residual energy on you, or just use more powerful cleaning mantras or words.