The increasing frequency of flash crashes in the $5.1 trillion-a-day foreign exchange market has regulators scrambling for answers. : LONDON/NEW YORK (Reuters) - The increasing frequency of flash crashes in the $5.1 trillion-a-day foreign exchange market has regulators scrambling for answers. Sudden, violent and often quickly reversed price moves are now a regular occurrence in world currency markets — often during the so-called ‘witching hour’, a period of thin trading between 5-6 pm in New York when currency dealers there have powered off and colleagues in Tokyo have yet to sign on.