When the global economy went into hibernation to try to halt the spread of Covid-19, companies everywhere scrambled to get their hands on cash. Banks, shareholders, and governments were all tapped for money, but self-help played a big part too. Businesses tried to clear their inventories, to get paid quickly for their products and services, and to pay their bills more slowly. A company that does all those things should have more funds in the bank to pay wages and other fixed costs—crucial if you want to survive an economic crisis of this scale.

As is so often the case, those creative types in the finance industry have come up with a smorgasbord of ways to help firms increase their cash holdings. The oldest and most common is called factoring. Essentially, if you're a cash-strapped company whose customers are dragging their feet on paying their bills, no problem: A bank will give you an advance on those invoices, for a fee. Another increasingly fashionable technique is a more complicated service known as reverse factoring or supply-chain finance. This allows a company's suppliers to quickly get paid what they're owed. The company then refunds the finance provider at a later date.

Both of these techniques—known as "invoice financing"—generate chunky fees for banks and fintechs such as Greensill Capital, a reverse-factoring specialist backed by SoftBank's Vision Fund. Viewed positively, these arrangements help keep the wheels of commerce oiled, especially at a time when supply chains face massive disruption—and uncertain payment schedules—because of the pandemic. In theory, they let everyone get their money reasonably quickly. However, this is really just another kind of short-term borrowing, and one that's not well-understood by investors. It can lead to nasty surprises.