KCG Holdings swung to a profit in the fourth quarter, as market volatility spurred trading volume, especially in equities.

KCG was formed when Knight Capital Group merged with Getco in July 2013. As a high-frequency trading firm, it uses powerful computers to zip in and out of markets, earning tiny profits on hundreds of thousands of transactions a day. However, KCG and other such firms have been on the defensive, as state and federal regulators have launched investigations into their practises.