Declared short sales can only be made on a price uptick from the previous trade.  This is to prevent the continual smashing of a stock on a particularly bad day.  This rule has been around for a very long time & in theory would be helpful on a day like 9/11.

The SEC actually changed this rule about six months ago and no longer requires an uptick in price in order to short.  Seems like a dumb move and there have been some comments that this is one reason for the increase in volatility in the markets.