U.S. securities regulators have taken the unprecedented step of asking high-frequency trading firms to hand over the details of their trading strategies, and in some cases, their secret computer codes.
Trading code is a high-stakes secret for high-frequency firms that battle each other to earn razor-thin profits on tiny price imbalances in the market. Such firms can make thousands of trades per second and provide much liquidity to the market.
High-frequency trading is estimated to be involved in more than half of all U.S. stock trading. Regulators have said the algos behind such trading were a factor in the flash crash, but that they did not cause it.
https://business.financialpost.com/2011/09/02/sec-takes-aim-at-high-frequency-traders/