Were they using it as a back up for rappelling? Or just as an ascender? I've ascended like 180ft using a shoulder length dyneema as a prussic and had no problems to unstick a rope. I can see it getting hot rapping with it but not as an emergency ascender.
By Leo Paik Administrator From: Westminster, Colorado Dec 30, 2013
Tristan, they used them as ascenders in situations where we didn't plan on ascending using them. They were slings for leading. They ascended or tried to ascend vertical/overhanging terrain, they didn't hold, they slid down the rope using them...much to their chagrin.
To clear up the confusion, the kleimheist/prussic was used only to capture progress when pulling through the "uphill" side. All weight was supported by a locking biner through the belay loop. The sling was left loose and was pushed forwards by the biner as one traversed. A microtraxion with biner backup would have been ideal, but we made do with what we had at the time.