Volume is the fuel of the market, since stock prices only move up or down when shares are trading hands. Most stocks trade hands at an even pace for days or weeks at a stretch until special events occur. Those events might be rather ordinary, such as an announcement of earnings, a new product or new executive. Or they might be extraordinary, such as a merger or a new corporate alliance.

Sometimes, however, trading volume spikes upward and the price moves higher for no apparent reason. Very often it ultimately turns out that the reason was major purchases of the stock by large institutions, hedge funds, mutual funds or private investors. The stealthy entry of strong players into a stock with enough buying power to boost volume by more than 200% and buoy the price is bullish for current investors. These major players are generally more knowledgeable and critical about their investments than individual investors -- and their entry at a particular price tends to set a floor for the future trading of a stock.

By itself, however, a price jump on big volume is not a reason to buy or hold a stock. It is just a clue that the stock is likely to be under accumulation by major players. Occasionally, stocks that rise fast when bought by price-momentum traders -- that is, investors who buy stocks simply because they are going up, trying to catch the trend -- can go down equally fast when bad news hits.

To determine whether the stock is under accumulation by big traders more attracted to the company’s fundamental story, check Form 13-D and Form 13-G SEC filings in Stock Research. Any investor or fund company that becomes the "beneficial" owner of more than 5% of a stock must tell the government -- and, by extension, you -- within 10 days.