Here is the deal: Funds lend some of their stocks and bonds to Wall Street, in return for cash that banks like JPMorgan then invest. If the trades do well, the bank takes a cut of the profits. If the trades do poorly, the funds absorb all of the losses.
The strategy is called securities lending, a practice that is thriving even though some investments linked to it were virtually wiped out during the financial panic of 2008. These trades were supposed to be safe enough to make a little extra money at little risk.
JPMorgan customers, including public or corporate pension funds of I.B.M., New York State and the American Federation of Television and Radio Artists, ended up owing JPMorgan more than $500 million to cover the losses. But JPMorgan protected itself on some of these investments and kept millions of dollars in profit, before the trades went awry.