Say the the BOJ loses control in Japan and Nikkei crashes, while all all markets crash as well. If the S&P starts to look like it's ready to free fall, the Fed will immediately step in to increase QE. If the S&P does not respond, QE will increase yet again and again until S&P behaves like a good dog. The net result would be all stock markets around the world can be down 50 to 80%, whereas the S&P maybe even mildly in positive territory. Remember, deleveraging is verboten here.