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Re:A High Frequency Trader's Apologia, Part 1 by dpark on Tuesday April 17, 2012 @02:25PM (#3854194)
The SEC seems to disagree, and says that HFTs added to the drop. HFTs also apparently burned through about half of the trading volume just trading with each other.

> The combined selling pressure from the Sell Algorithm, HFTs and other traders drove the price of the E-Mini S&P 500 down approximately 3% in just four minutes from the beginning of 2:41 pm through the end of 2:44 pm. During this same time cross-market arbitrageurs who did buy the E-Mini S&P 500, simultaneously sold equivalent amounts in the equities markets, driving the price of SPY (an exchange-traded fund which represents the S&P 500 index) also down approximately 3%.

> Still lacking sufficient demand from fundamental buyers or cross-market arbitrageurs, HFTs began to quickly buy and then resell contracts to each other – generating a “hot-potato” volume effect as the same positions were rapidly passed back and forth. Between 2:45:13 and 2:45:27, HFTs traded over 27,000 contracts, which accounted for about 49 percent of the total trading volume, while buying only about 200 additional contracts net.[9]

Re:A High Frequency Trader's Apologia, Part 1 by nihilum on Wednesday April 18, 2012 @03:24AM (#3856907)
The SEC paper doesn't even use the word "liquidity" properly, and the authors appear to be biased.

http://www.nanex.net/aqck/2977.html