One other aspect of HFT that was not mentioned in the article is that HFTers often seek arbitrage opportunities. For example, the value of many ETFs such as SPY (i.e. an ETF tracking the S&P 500) are derived from the value of underlying securities. If the value of SPY versus the value of the underlying securities becomes out of sync, HFTers may go long one and short the other and then profit when they converge again. In this sense, HFTers only profit if the market returns to fair value. This applies to many ETFs, convertible securities, and options.