These exotic financial instruments conveyed information crucial to seeing the 2008 financial crisis in advance. That knowledge allowed astute speculators to get positioned for massive profits as the crisis unfolded.

In the coming crisis—which has already started—I expect CDS will again play a key role in telegraphing important information shrewd speculators can use to their advantage.

A CDS is a contract between two parties. Think of it like an insurance policy against a borrower—typically a large company or a government—defaulting. One party underwrites the insurance policy, and another buys it. If the borrower defaults, the CDS issuer pays out the CDS buyer.

CDS trade in the open market and reflect investor expectations of the default probability of a particular borrower. The more likely the underlying entity is to default, the more expensive the insurance (CDS) will cost.

The seller of the CDS collects a premium and bets that the underlying entity will not default. Conversely, the buyer of the CDS is betting that the underlying entity will default or become more distressed so that he can sell the insurance policy in the market for a higher price than he paid it for.