AIUI the answer is basically "it's very hard to prove", which is why very few people actually get punished for it. IIRC in the flash crash trader's case he actu

AIUI the answer is basically "it's very hard to prove", which is why very few people actually get punished for it. IIRC in the flash crash trader's case he actually had emails that made it very clear what his intentions were with the orders he placed (but I could be misremembering).