Rope Rescue Systems Testing Main, Belay ( Back Up) Failures

Test comparison between Two Tensioned Rope Rescue System and Dedicated Mainline with UnTensioned Belay (backup) Rope Rescue Systems. Challenging the premise of requiring an untensioned belay line (backup) as opposed to twotensioned techniques. Is there any evidence to support the use of an untensioned belay (backup) in rope rescue