"In layman’s terms, we can think of a swap line as a standing guarantee of U.S. dollar liquidity. If you (as a central banker) ever need greenbacks in a pinch, you know you’ll be able to procure them instantly, no matter how “tight” the open market may be.
This standing guarantee reduces the odds of another violent $USD spike of the type we saw in late 2008. In a way, one can think of it as “short squeeze insurance.”
The many players around the world who are “short” U.S. dollars — by way of lending arrangements denominated in dollars and so on — have spiking dollar risk implicit in their positioning.
What the Fed has essentially said to these players is, “It’s okay for you to keep borrowing in dollars, because in the event of a new liquidity crisis we will create accessible dollars for you (via the channel of your local CB).”" Read more. Fed Swap Lines Purposefully Keeping Dollar WeakMercenary Trader
Seeking Alpha, December 22, 2010. Image by Idea go / FreeDigitalPhotos.net.