Mario Draghi is becoming one of currency traders' only friends.

With the $5.3 trillion-a-day foreign-exchange market poised to deliver its worst first-half returns on record, the carry trade is about the only way traders are making money by exploiting differences in global borrowing costs as volatility tumbles. That strategy became more profitable after the European Central Bank president cut interest rates on June 5.

"The ECB has signaled risk is on again," Eric Busay, a Sacramento-based money manager at the California Public Employees' Retirement System, the largest U.S. public pension fund with $294 billion in assets, said in a June 6 phone interview. "People are concerned when to exit the trade and they understand the rush to exit could be crowded. But at the same time, you have to be in it to win it."