Oh trust me I do that, but it pretty much never does any good. If things are that close to the edge it's generally due to a lackluster back line in the first place and they won't get to me in those 6 seconds. I did once get to play emergency tank in Divine Might II on Normal. That worked out because the healers were able to focus solely on me during the recovery stage. Cureskin helped me to keep my shadows up. That sort of thing is by far the exception though.