I think it was that this guy traded to "cause" instability, by placing bogus futures trades that were basically the value of the entire order book for those fut

I think it was that this guy traded to "cause" instability, by placing bogus futures trades that were basically the value of the entire order book for those futures. If you have a Level II stock data feed, you can view the order book for particular stocks, and similar data is available for futures from CME group. As an example, a friend of mine day trades Tesla stock (& options) and basically a swing trade, has figured out if the stock cycles down by a few cents, and they put a relatively large order at a certain price. The market sees the trade with a higher price and the stock increases. This has resulted in some large profits for swing trades, somewhat due to believing in Tesla's fundamentals and in Elon Musk. Some institutional investors don't share my friends views, and trade Tesla stock down. It is legal because the trade was made in good faith, and not trying to "deceive" the market using bogus trades. In traditional HFT, the legal justification for fast trades that in some cases were never intended to be filled, is murky at best. As long as the market stays reasonably stable and the big boys profit, the complaints are somewhat muted. A friend in the same group, who also trades Tesla, was on the console during the flash crash. It was "obvious" that these automated trade bots were unloading, and there was not major news (eg. GM bankruptcy) to justify the sudden price drop in specific stocks. Her console showed "black swan" type data, and she switched to an option trade as stocks caught up in the flash event wouldn't stay down for long. The options that were "out of the money" (almost worthless) became valuable when the stocks rebounded. What caused the actual flash crash (triggered by futures trades or not) was a phenomenon called "exponential backoff" with a large automated trading bot closing out its trades, triggered by stocks below a certain price, causing downward pressure on the stocks, and the market. Another automated trade bot sees this, and also closes out its position. This automated close out then occurs exponentially, and the market goes into a "death spiral" crash. Note: I do not own stock in Tesla. I'm not a stock broker and this is not investment advice.