What that means in practice is that some people will buy after 5% drop, more after 10%, even more after 15%, etc... I don't believe anymore that we can have a proper crash in this situation. If we do, the price will bounce back in a matter of seconds. I'm probably not the only one keeping an open order to buy as many bitcoins as possible at below £5. Lack of sell-stop orders on mtgox also means there's nothing to balance them out (apart from automatic trading).