Essentially, some funds use investor money to buy certificates that represent an unsecured claim with a counterparty. Because the claim is unsecured, and because the claim is rarely exercised, the counterparty may never actually hold enough gold to pay off all the claims at once. [Some might notice that this was precisely one of the origins of fiat money hundreds of years ago.]
Now imagine that investors decide to redeem their fund certificates for gold all at once (i.e. a run on the ‘bank’)…
Alchemists Turn Paper Into Gold

