Using transaction level data in the E-mini market during May 2-6, 2010, Kirilenko et al. (2011) study the behavior of HFTs during the crash. As we mentioned, HFTs act similarly to traditional market makers and when market conditions are normal, they provide liquidity to the market. In contrast to traditional market makers, however, HFTs trade aggressively when prices are about to change in an effort to keep their inventories down. This behavior was apparent on May 6, when the speed of their trading algorithms allowed them to unwind their inventories quickly before the price decline of the E-mini contract accelerated. As a result, they aggressively drained liquidity from the market, amplifying selling pressures on the E-mini market. The authors conclude that “although HFTs did not trigger the ‘flash crash,’ their responses to the unusually large selling pressure exacerbated market volatility.” The increase in trading aggressiveness on May 6 is also documented by Chakravarty et al. (2011), who show a sharp increase in the use of Intermarket Sweeping Orders, a particular type of order that allows a trader to “sweep” liquidity across all market centers simultaneously.