When you've been approved for margin stock trading, you're also eligible to short stock. Almost every successful stock trader has shorted stock at one time or another. When you short stock, you make money when the company's shares fall—or, even better yet, when they crash. The problem is that you can expose yourself to unlimited liability when you do this.
Barchart calculates a custom index (the Barchart Market Momentum Index) which is used as an indicator of overall change in the markets. The index reflects the movement of more established stocks: those with SEC filings, with 6 months of trading activity, and with price above $2.00. A high value means the market is moving up, where as a negative value implies the market is down today.