A Dutch pension fund or Japanese insurer could use borrow dollars then lend them as euros or Japanese yen, before later repaying them.
The FX swap market has a history of problems, including funding squeezes during both the global financial crisis and again in the early days of the Covid pandemic, before the Federal Reserve raced to the rescue.
Terrifyingly, the $80trillion-plus "hidden" debt estimate is greater than the total stocks of US dollar Treasury bills, repo and commercial paper in circulation combined, BIS said.
In other words, it's bigger than the mighty dollar.
It has grown from $55trillion to $80trillion in a decade, with daily FX swap deals totalling a massive $5 trillion a day.
Non-US banks and pensions funds have twice as much FX swap dollar obligations as the amount of dollar debt that is listed on their balance sheets.