High-frequency trading is carried out by powerful computers that use complex algorithms to analyse markets and buy or sell shares within seconds. As the name suggests, speed is key and firms can gain an advantage by moving milliseconds earlier than their competitors. Most high-frequency trading is carried out by investment banks and hedge funds using automated trading platforms, but there are also high-frequency trading firms dedicated to the craft. It is not clear which hedge funds were involved in the Bank of England breach. The algorithms can find new trends across global markets and trade on them automatically before other players have a chance to catch on. The computers will place large volumes of trades across different markets in order to increase profitability on transactions that would otherwise have very small profit margins owing to the marginal movements in share or currency markets that the trades are seeking to capitalise on. So size and speed is how they make their money...