The mere expectation or rumor of a central bank foreign exchange intervention might be enough to stabilize the currency. However, aggressive intervention might be used several times each year in countries with a dirty float currency regime. Central banks do not always achieve their objectives. The combined resources of the market can easily overwhelm any central bank. Several scenarios of this nature were seen in the 1992–93 European Exchange Rate Mechanism collapse, and in more recent times in Asia.
Just like stocks, you can trade currency based on what you think its value is (or where it's headed). But the big difference with forex is that you can trade up or down just as easily. If you think a currency will increase in value, you can buy it. If you think it will decrease, you can sell it. With a market this large, finding a buyer when you're selling and a seller when you're buying is much easier than in in other markets. Maybe you hear on the news that China is devaluing its currency to draw more foreign business into its country. If you think that trend will continue, you could make a forex trade by selling the Chinese currency against another currency, say, the US dollar. The more the Chinese currency devalues against the US dollar, the higher your profits. If the Chinese currency increases in value while you have your sell position open, then your losses increase and you want to get out of the trade.
One unique aspect of this international market is that there is no central marketplace for foreign exchange. Rather, currency trading is conducted electronically over-the-counter (OTC), which means that all transactions occur via computer networks between traders around the world, rather than on one centralized exchange. The market is open 24 hours a day, five and a half days a week, and currencies are traded worldwide in the major financial centers of London, New York, Tokyo, Zurich, Frankfurt, Hong Kong, Singapore, Paris and Sydney—across almost every time zone. This means that when the trading day in the U.S. ends, the forex market begins anew in Tokyo and Hong Kong. As such, the forex market can be extremely active any time of the day, with price quotes changing constantly.