You can see a chart of the after hours action by hitting "e" on the Google Finance chart (you may have to click on the chart first):

https://www.google.com/finance?chdnp=1&chdd=1&chds=1&chdv=1&...

Example: say BigTechCo will be releasing earnings reports a week from now. BigTechCo stock slowly rises in anticipation of a good report. A week later, BigTechCo announces that every possible metric is better than ever, and the outlook is even better for the next quarter/year/whatever! Analysts are pleasantly surprised, and everyone is 100% on board in believing the numbers and the rosy outlook. Within 48 hours, the stock tanks as everyone furiously sells off. It happens all the time and makes zero sense to me. I'm sure there are reasons I'm not aware of, but I've got to think it's from some sort of irrational group behavior as well.