One way this happens. They're not only selling but buying at the same time. Great article!
Shorts need to control the stock's share price over a long time (often several months to well over a year), and can't afford to just accumulate an unlimited number of short positions in the stock, so they have to be buying shares at the same time they are selling shares too. When the shorts drive the share price down, eliminating buyers as discussed above, some of those investors trying to sell their shares at that same time will follow the share price activity downward lowering their ask price.
Now the shorts can buy back some of the shares they have shorted at lower prices including some shares where longs have put stop-loss sale orders to protect against downside losses. The shorts will only buy shares part of the way back up as the share price rises, and then wait to see if new buyers come into the market. If the share price continues to rise up to much again during the day, the shorts will repeat the same selling and buying process to control the share price.
http://m.seekingalpha.com/instablog/2918951-g-hudson/1026551-how-the-big-players-manipulate-the-stock-market
