The Fed released the news that it was not going to taper its easing efforts in a very specific manner, using a lock room to prevent the news from being released even via the media prior to the minute it wanted.
With certain “news” firms specializing in delivering market-moving news directly to black boxes and traders, trading begins (almost) immediately following the release of news – depending on how long it takes for the information to reach the players.
Even at the speed of light, though, there’s a considerable delay between a firm in NYC getting the news, and a firm in Chicago, but that delay did not occur following the Fed’s announcement here:
It’s not exactly clear how the information got to Chicago markets so quickly. But the Federal Reserve is concerned enough about the unusual event that it has begun discussions with news organizations.