Speculators are able to swing large amounts of money in and out of foreign currencies by selling insurance. In stock markets the insurance is known as a derivative. The call allows me to buy a specified number of Hong Kong dollars for a specified number of US dollars over a fixed period. The rise of this derivative market has allowed world trade to boom by allowing importers and exporters to buy insurance against currency risks, or the risks in the prices of commodities. What happens in the call market influences currency markets and interest rates because calls are hybrids of bonds and currency.
Calls allow speculators to borrow on margin and lever their hunches into large currency movements.
Their reward for a successful guess is that they get to keep their insurance fee. The people who bought the insurance have about as much regret as do car drivers who don't crash by the end of the year, and so fail to cash in on their insurance premium. The only parties bothered by these private insurance deals are naughty governments. Derivatives are modern financial instruments that allow investors to rapidly discipline misbehaving politicians. Soros' proposal to slow down capital markets appeals to world leaders.