If you are using ISBoxer, I do not recommend a button solely for moving backwards. Instead, many of use have a 2 step button that does 1 action while the button is held down, then preforms an action once the key is released.

On the way down, have teh key set for Interact-with-target; on the way up, have it set to go back.

If you press the key, as long as it's held down, it will move toward the target, and once you release, it will stop moving, but still be facing the target. This is how you can make snared casters turn to face a moving target that is behind them.

This is a more useful key to break follow because it also incorporates IWT.