So how did we get here, and just how much should the non-currency traders among us worry?

1. It actually began before the financial crisis. Capital gushed into emerging markets—particularly commodities—between 2004 and 2008, even as the Fed raised rates from 1 to 5.25 percent. It was BRICs euphoria, and nothing could stop it. Nothing except Lehman. But once it became clear the world would not, in fact, end, capital went back overseas to rekindle the old magic (and find better returns than the Fed's zero percent interest rates).* It went back to emerging markets. Higher growth and higher interest rates there looked like no-brainers compared to the low growth and low rates in the at-best-moribund U.S. and Europe.

2. That left the emerging markets with a dilemma: They could raise rates and get a foreign credit boom, or cut rates and have a domestic credit boom. If their central banks kept, or raised, rates high where they "should" have been, it would have only attracted more "hot money"—quick, speculative capital looking for the best return—from abroad. This would have made their exports even less competitive by pushing up their currencies more, and set off a lending boom that could reverse itself at the click of a mouse.

But it was a bit of a catch-22. If their central banks kept rates lower than they should have, it would have made their economies less attractive to yield-hungry foreign investors. Less capital would have flowed in, and their exports wouldn't have been as priced out by a too-high currency—but persistently too-low rates would have risked inflation and a credit boom of their own making.

They chose door number 2. They chose it, because they had seen what hot money could do to an economy when it got scared: the East Asian financial crisis had created mini (and in Indonesia's case, not-so-mini) depressions back in 1997. Instead, emerging markets decided to try to keep their currencies low to run big trade surpluses, and build up rainy day funds of foreign reserves. So when the Fed lowered rates in 2003, they did too. The same in 2008. But, as you can see in the chart below from the World Bank, these low rates that made sense for the U.S. didn't make sense for emerging markets. They were too low.

3. But there was a door number 3. Emerging markets could have stopped hot money from coming in. Such "capital controls" would have let them raise rates as needed without worrying about foreigners pushing up their currencies or blowing bubbles. Indeed, China uses them, and Brazil kind of did too with its now-defunct financial transactions tax. But most emerging markets didn't. Capital controls were too heretical, just not what you were supposed to do (even though now even the IMF thinks they're okay).

4. So emerging markets had big domestic credit parties that started to run out in 2011. But then foreign money—yes, some of it from QE—kept the party going awhile longer. That ended last May, though, when Bernanke said the Fed would soon start drawing down QE. Hot money ran for the door, currencies dropped, and the weakness that had been there all along became obvious. Particularly, Barry Eichengreen found, in countries that 1) were borrowing from abroad, and 2) had big enough financial markets that it was easy to sell.