This is not entirely true. When you short sale you are still essentially selling shares (although you don't own them). Someone else is still buying these at the price you sold for. If enough people short a stock it will act as heavy sell pressure and drive the price per share down just as it would if everyone decided to dump their existing shares. You see pps go up a lot when all of the shorts start covering because they have to "buy" the shares they sold back causing buying pressure.