Foreign exchange intervention has long been one of those things that works better in practice than in theory.*
Emerging markets worried about currency appreciation certainly seem to believe it works, even if the IMF doesn’t.**
Korea a few weeks back, for example.
Korea reportedly intervened—in scale and fairly visibly—when the won reached 1090 against the dollar in mid-August:
“Traders said South Korean foreign exchange authorities were spotted weakening the won “aggressively,” causing them to rush to unwind bets on further appreciation. On Wednesday (August 10), according to the traders, authorities intervened and spent an estimated $2 billion when the won hit a near 15-month high of 1,091.8.”
And, guess what, the won subsequently has remained weaker than 1090, in part because of expectations that the government will intervene again. And of course the Fed.
And that is how I suspect intervention can have an impact in practice. Intervention sets a cap on how much a currency is likely to appreciate. At certain levels, the government will resist appreciation, strongly—while happily staying out of the market if the currency depreciates. That changes the payoff in the market from bets on the currency. At the level of expected intervention; appreciation becomes less likely, and depreciation more likely.***
1090 won-to-the-dollar incidentally is still a pretty weak level for the won, even if the Koreans do not think so. The won rose to around 900 before the crisis, and back in 2014, it got to 1050 and then 1000 before hitting a block in the market. In the first seven months of 2016, the won’s value, in real terms, against a broad basket of currencies was about 15 percent lower than it was on average from 2005 to 2007.