The boundaries of yesterday's low and high marked an important level in many global markets. On Sensex/Nifty yesterday's low is of significance. It was not just a 'hammer' pattern - the low also came of an important trendline.
The trendline is more perfect if drawn on the Sensex, we can connect the lows of March 2009 through these significant lows. A close below yesterday's low can make things turn nasty and that makes yesterday's low a must hold.