The principle mistake that many commentators make is the assumption that central banks can separate the currency allocation of reserves from their exchange rate objectives. In practice, this is often not the case, especially for the large surplus economies in Asia as well as the oil-exporting countries. These countries all follow some sort of dollar standard, whether it is an outright peg or a dirty float.

So, when they intervene to prevent their currencies from appreciating against the dollar, they get mostly – or even exclusively – dollars (also because most of their trade and capital flows are dollar denominated).

However, it is difficult to sell those dollars back to the market without causing renewed dollar weakness and, thus, trigger new interventions. Some small central banks may get away with it, but not the group of large reserve holders. There is no free lunch: if you shadow the dollar you also have to hold it.