"The ease with which folks such as Mr. Spirou were borrowing against their homes (from WAMU) sent warning signals to some investors. The concern: that in their rush to attract customers amid the housing boom, mortgage lenders were lowering their standards. In 2005, for example, the share of interest-only loans grew to about 18% of all subprime loans, from next to nothing in 2001, according to data provider First American Loan Performance. Over the same period, the share of subprime loans that required little or no documentation of the borrower's income grew to more than 16% from about 10%.
"We looked at this and thought we're going to see lenders who are misusing these tools," says Mr. Whalen. "We're going to see the performance of their bonds deteriorate."
At about the same time, in early 2005, Wall Street bankers were developing a new kind of derivative contract that would allow investors such as Mr. Whalen to make bets based on their misgivings. Called a credit-default swap, it had previously been applied mainly to corporate and sovereign bonds. Like an insurance contract, it pays off if a subprime-backed bond suffers a certain amount of losses to defaults. The holder, known as a protection buyer, makes regular payments to a bank or other counterparty for the insurance, and also has the right to resell the contract. If defaults prove higher than expected and the bond starts to look riskier, the value of the contract rises, and the holder can resell it at a profit."
http://www.realestatejournal.com/buysell/mortgages/20061031-whitehouse.html