Instantly markets spiked lower, the most notable being the SP500 index free falling by 1% before recovering 3 minutes later. If you are wondering how traders could keep up with the "news", analyze the effect and place relevant trades instantly well the sad truth is they don't. Most trading firms use computer trading software to read trusted news and market participant's: tweets, newspapers, websites, etc. The software then analyzes that data and based on weighted outcomes of that news places trades without reference to human traders. So this is how one tweet, from a trusted source, can be read, analyzed and a trading strategy formed and executed in the space of a second if not less. Note the best of these programs can execute individual trades measured in picoseconds ( 10−12 of a second).
One has to wonder if the hackers had just out of the money puts on the SP500 index waiting to capture the drop?
According to zerohedge 26,000 SP 500 e-mini contracts traded in 3 mins, see chart below: