Magnetar solved a conundrum of those who bet against the market. An investor might be confident that things are heading south, but not know when. While the investor waits, it costs money to keep the bet going. Many a short seller has run out of cash at the gates of a big payday.
...Even today, bankers and managers speak with awe at the elegance of the Magnetar Trade. Others have become famous for betting big against the housing market. But they had taken enormous risks. Meanwhile, Magnetar had created a largely self-funding bet against the market.
Full article: http://www.propublica.org/feature/all-the-magnetar-trade-how-one-hedge-fund-helped-keep-the-housing-bubble