According to bankers and others involved, the Magnetar Trade worked this way: The hedge fund bought the riskiest portion of a kind of securities known as collateralized debt obligations -- CDOs. If housing prices kept rising, this would provide a solid return for many years. But that's not what hedge funds are after. They want outsized gains, the sooner the better, and Magnetar set itself up for a huge win: It placed bets that portions of its own deals would fail.