Benzodiazepines work through the limbic system in our brain, which is - among other areas of responsibility - responsible for awakeness and falling asleep. But when benzodiazepines shut down your limbic system, your motoric system is not shut down and continues to be operational. As a result of that, people were observed and reported to have been walking and doing things while they were unconscious due to the benzodiazepines (similar to sleepwalking). That is why this class of drugs is just about perfect for drug cocktails (since you will not be conscious to feel any pain), but not so for any method which requires for our motoric system to be shut down (like this method, since the motoric system has to be shut down so that one does not remove the bag from one's head, due to the feeling of suffocation).