uptick rule: in order to short the securities the last change in price must have been a rise in that price... (in other words, you can only short if the security is on the way up ...traditionally by at list a 'teenie' or a 1/16 ...ofcourse today we use decimals)
however: unfortunately the SEC scrapped this rule a while ago.
this is part of the reason why the MARKETS are in such a spiral downward ...it's so easy to short ...you just short as the security is tanking ...everyone jumps on ...consequently you see the like of BEAR STRENS and goldman sachs.
we will see the 'up-tick rule' come back along with many other regulations.
cheers ...danny