Traders at the banks would collaborate in online chat rooms. One trader would agree to build a huge position in a currency, then unload it at 4 p.m. London Time each day. That's when the WM/Reuters fix price is set. That price is based on all the trades taking place in one minute. By selling a currency during that minute, the trader could lower the fix price. That's the price used to calculate benchmarks in mutual funds. Traders at the other banks would also profit because they knew what the fix price would be.
Forex is a portmanteau of foreign currency and exchange. Foreign exchange is the process of changing one currency into another currency for a variety of reasons, usually for commerce, trading, or tourism. According to a recent triennial report from the Bank for International Settlements (a global bank for national central banks), the average was more than $5.1 trillion in daily forex trading volume.