On a good day, 27-year-old Bobby Timberlake at CME Group Inc. in Chicago rounds up $2.5 billion from the world's biggest traders and banks such as JPMorgan Chase & Co. to cover their losses in the $639 trillion derivatives markets.

What happens on a bad day will test new rules in the Dodd- Frank Act designed to prevent a repeat of 2008's credit crisis. Starting in March, as much as 79 percent of derivatives trades known as swaps must be backed by collateral and go through clearinghouses such as CME Group. Traders may have to post $927 billion with Timberlake and his peers at LCH.Clearnet Group Ltd. and IntercontinentalExchange Inc., whose role as middlemen is to ensure participants get paid.

This arrangement can withstand almost any shock, including defaults by four of the biggest lenders, according to the clearinghouses. Some bankers and researchers aren't convinced. They warn unprecedented amounts of risk will be concentrated in a handful of clearinghouses — some newly eligible for emergency Federal Reserve loans. If they fail, taxpayers who financed $1.2 trillion of bailouts last time could be on the hook again.