Exchanges are scrambling to cater to the high-frequency crowd, in part because the huge volumes they generate can make it easier for other traders on the exchange to execute orders.

WSJ: Ban on Flash Orders Is Considered by SEC

Some investors use ETFs as a quick way of expressing their overall view on the market, while high-frequency traders use the funds as part of their complex arbitrage strategies.

ECONOMIST: The risks created by complicating a simple idea

Direct Edge and Bats have successfully siphoned double-digit volume away from Nasdaq OMX and NYSE Euronext in recent years, in part by appealing to high-frequency trading firms with fast execution systems.

FORBES: Birth Of A Stock Exchange

Part of the problem on May 6 was some of the prominent high frequency traders, who normally contribute bids and offers continuously all day long, stopped trading and thus liquidity vanished.

FORBES: High Frequency Traders Face SEC/CFTC Panel

Another part of the problem: Many traders, including a number of high frequency trading firms that normally make up 60% of daily trading volume, stepped aside and stopped buying, draining liquidity from the system when it was trying to deal with a tidal wave of sell orders.

FORBES: SEC Sets 6-Month Test of Stock Circuit Breakers