How fast? I haven't quite been able to determine. As an example, consider the rather drastic behavior today of EXPE: As you can see, the bottom pretty much fell out at approx. 11:13 am ET. But the closest dated relevant news story I could find came out some 24 minutes later: Source: http://biz.yahoo.com/ap/070615/expedia_mover.html?.v=1 Something triggered over half a million shares being dumped in a minute or two. If anyone has a better news feed and can turn up the story or can provide an alternate explanation, I'd be very interested in it. Full disclosure: Yes I was long on EXPE and had a nice little profit going when this happened. Fortunately, it wasn't a very large position, so I'll recover from the hit. But it still stung, and in this case almost seemed unfair. Yeah, I know, I should have had a stop set. But this kind of drop is completely beyond my experience. In ever other trade I always had enough of a moment to assess things and keep a pretty tight loss. But not this time.