When foreign-exchange broker FXCM was forced to seek a rescue last week following a surprise surge in the Swiss franc, a number of potential white knights surfaced, including private equity firms and Leucadia National.

It soon became apparent, however, that the best-suited among them to pull off the lightning-fast cash infusion FXCM needed was Leucadia and its Jefferies unit, which in recent years has honed its skills as a rescuer of firms succumbing to bouts of market turmoil.