When positive momentum grips a stock, each high is succeeded by a higher high. Investors seeing a stock hit 150 buy, because they're convinced that the stock is headed to 200. Any dip becomes a buying opportunity that leads to a new high. When negative momentum grips a stock, each low is succeeded by a lower low. Investors seeing the stock hit 100 decide to sell because they're convinced the stock is headed to 50.
Any rally becomes a selling opportunity that leads to a new low.
You can see this negative momentum clearly at work in the chart of a stock like VerticalNet. The stock hit a high of nearly 139 at the close on March 9. I expect that there's more damage to come. This kind of negative momentum owes very little to any good or bad fundamental news about a company's revenue or earnings. Instead, look to the price history of the stock.