A central bank cannot create another country's money. When local banks cannot roll their dollar funding in the eurodollar-market, their own central bank borrows dollars from the Federal Reserve against its own currency, lends them domestically, and reverses the trade later at the same rate.
The Fed maintains standing lines with a small group of major central banks and has opened temporary ones more widely during stress episodes. Announcements of new lines have repeatedly coincided with a sharp narrowing of the cross-currency-basis and a topping-out of dollar strength.
For a trader the signal value is high: swap line usage and basis levels together indicate whether a dollar rally is a growth story or a plumbing story, which matters for how long it lasts.
Example: a swap line draws $100bn. The three-month basis narrows from minus 90 to minus 25 basis points within days, and the dollar's trade-weighted rally stalls.
Related: eurodollar-market, cross-currency-basis, reserve-currency, fx-swap