Here is a good summary posted by Mustbefree:
More often than not, news is known or anticipated well before it is released. The only type of news that generates runs now adays is news that was completely unexpected. More often than not, momentum and swing traders will buy in anticipation of news. Their buying helps drive the stock up. Many use the release of the news as their sell event. It's a very easy sell signal and and is fairly reliable. Another reason to sell on news is that volume typically increases. This gives a better chance to dump the stock and get top dollar from your sell. But if the sell pressure is a lot higher than the buy pressure, then the stock will get beat up in a hurry. It looks like the entire market was bought up in anticipation of the bailout news. Thankfully, many that sold on news will be looking to re-enter once the stock settles. Right now, people that bought on the news are bailing out thinking that they made the wrong choice (based on the stock action). Today's market is a much different creature.
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