Day Trading

I think you have the facts confused; it was in the past illegal to sell short on a down tick, and short sales could only be made on upticks or even trades after an uptick. The purpose of this rule was to limit persistent short sales for the purpose of driving down a stock's price. That rule was recently abandoned, and yes, I think it has had an effect on a few stocks, which have been shorted relentlessly by 'momentum' traders.