HFT firms which seek to exploit currency movements or arbitrage price differences between local-currency and foreign-currency bonds want to move funds in and out of a country with minimal friction.

That partly reflects the problems emerging-market Banks face in shifting excess funds into foreign-currency assets, or among subsidiaries in different countries.

The trade surplus pours a steady stream of foreign-currency export earnings into China, funds the central bank buys up in exchange for local currency and holds in its official reserves.