On the 95 model, it overrides the reverse speed limiter and enables AWD to work in reverse
If the engine dies when you put it in reverse, the reverse speed limiter module has failed and grounds the ignition to kill the engine - either replace the reverse speed limiter, disconnect the black wire from the module (disconnecting the black wire will eliminate the speed limiter, but the override button will still have to be pushed to enable AWD) or just hold the override button in when shifting into reverse.