I'm thinking unpleasant thoughts about ribcages shattering and organs rupturing. People are heavy. You both lived to tell the tale though, so presumably it's not as dangerous as it sounds to me. Any idea how it helps the back though? Mine's not in very good shape at all. I imagine this is one of those things that requires one or more of the participants to understand exactly what they're doing though, so I probably can't just grab a random friend and demand they plod along my spine for research purposes...