Many of the contracts sold by the algorithm were purchased by high-frequency traders
(HFTs). HFTs are computerized traders who buy and sell at high speed. They account for
more than one-half of the overall trading in today’s markets. The HFT programs detected
that they had amassed excessive “long” positions, meaning that they had purchased a large
amount of stock with the expectation that its price would rise. They immediately began to
sell these stocks aggressively, which in turn caused the mutual fund’s algorithm to accelerate
its selling. As the HFT and mutual fund programs traded contracts back and forth, they
created a “hot potato” effect, where contracts changed hands 27,000 times in 14 seconds.
Despite this frenzied trading, however, only 200 contracts were actually bought or sold. In
most cases, the same contracts moved back and forth between the mutual funds and the HFTs
in microseconds.