On September 19, the SEC banned "short selling" of financial stocks. Short-selling is the "art" of borrowing shares in a stock, selling them, and buying the shares back later; hopefully at a lower price, thus making a profit. This may also be described as betting against the market. The SEC banned short selling of many stocks on the theory that "unscrupulous" short sellers were driving the price of stocks down and damaging strong and reputable companies (such as Lehman Brothers, Fannie Mae, and Freddie Mac). Without the shady shorts interfering (so the theory goes), the stock market would go up.
Here is a brief evaluation of the efficacy of that new law:
The DOW closed around 11,400 on September 19. As I write, it is around 9700. A staggering loss of almost 15% in less than a month.
There are many reasons for this decline (rising unemployment, a decline in mortgage equity withdrawal, etc.). But the short ban made things worse by eliminating a solid base of stock buyers.
Although short sellers sell stock, they also borrow those shares, and they are forced to buy (or "cover") those shares back at some point. When the stock market drops, the shorts cover, which then drives the prices up.
With the ban on short selling in effect, the shorts are not there to cover as the market goes down.