Call it Uncle Sam's hedge fund. The rescue of the American financial system proposed by Treasury Secretary Timothy Geithner is, in all but name, a gigantic hedge fund. The government would lend vast sums to private investors to enable them to buy loss-ridden assets at discounts from banks with the prospect of making sizable profits. If that's not a hedge fund, what would be? The hope is that the $14 trillion U.S. banking system would expand lending if it could get rid of many of the lousy securities and loans already on its books.

Almost everyone thinks that a healthier banking system is necessary for a sustained economic recovery. Can the Geithner plan work? Maybe, though obstacles abound. One is political. Private investors may balk at participating because they fear populist wrath. If the plan succeeds, many wealthy people will become even wealthier. Congress could subject them (or their firms) to humiliating hearings or punitive taxes. Why bother? Another problem: Investors and banks may be unable to agree on prices at which assets would be bought. (Article continued below...)

But succeed or fail, Geithner's plan illuminates a fascinating irony. "Leverage"—borrowing—helped create this mess. Now it's expected to get us out. How can this be? It's not as crazy as it sounds. Start with the basics on how leverage affects investment returns.

Suppose you bought a stock or bond for $100 in cash. If the price rises to $110, you make 10 percent. Not bad. Now, assume that you borrowed $90 of the purchase price at a 5 percent interest rate. Over a year, the stock or bond still increases to $110, but now you've made more than 50 percent. You pay $4.50 in interest and pocket a $5.50 gain on your $10 investment. Note, however, that if the price fell to $95, you'd be virtually wiped out ($4.50 in interest paid plus $5 lost on the security).

Economist John Geanakoplos of Yale University argues that the economy regularly experiences "leverage cycles." When credit is easy, down payment terms are loose. Investors or homeowners can borrow much of the purchase price of houses and securities. Prices of assets (stocks, bonds, real estate) rise, often to artificial levels because investment returns are so attractive. But when credit tightens—government policy shifts or lenders get nervous—the process reverses. Prices crash. Leveraged investors sell to repay loans. New borrowers face stiff down payment terms.