Wall Street's sharp downturn beginning in October has pulled the Nasdaq composite index into what's known as a bear market . The benchmark S&P 500 index is in what Wall Street calls a "correction" and is just a tick shy of a bear market, threatening to end the more than nine-year U.S. bull market run.For example, an investor shorts 100 shares of a stock at $94. The price falls and the shares are covered at $84. The investor pockets a profit of $10 x 100 = $1,000. If the stock trades higher unexpectedly, the investor is forced to buy back the shares at a premium, causing heavy losses.