The foreign exchange markets are famously the most liquid in the world, with $5 trillion or more being traded each day. But, increasingly, when that liquidity is most needed it has a tendency to vanish, with markets freezing and leaving investors unable to trade.

Changes in regulation and market structure have led to concerns that the market's shock-absorbing capacity is in decline. That is a worrying dynamic for a market that corporations and investors around the world depend on to manage their exposure to foreign currencies.