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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. 
Momentum investing usually involves a strict set of rules based on technical indicators that dictate market entry and exit points for particular securities. Momentum investors sometimes use two longer-term moving averages, one a bit shorter than the other, for trading signals. Some use 50-day and 200-day moving averages, for example. The 50-day crossing above the 200-day creates a buy signal. A 50-day crossing back below the 200-day creates a sell signal. A few momentum investors prefer to use even longer-term moving averages for signaling purposes.