The Central Banks of the world are among the biggest players in the Forex markets. They buy and sell their country’s own currency to influence it’s value for the country’s benefit. All of them do this in some way or form. It doesn’t always have the intended effect, and it can also piss off others who are affected by the move.
Take what happened earlier this month. The Swiss Central Bank sold of the Swiss Franc (it’s national currency) in droves to drive down it’s value. Amongst other things, exports were taking a hit because the value of the Franc was strong. This was viewed as an inherently selfish act by the rest of the Europeans on the single currency, the Euro. Devaluing the Swiss Franc (against the Euro) would have the opposite effect on them…stronger Euro, bad for THEIR exports.
There are all sorts of examples, from the US to China, where these Banks do their thing. Here is a video I found that highlights how this works: