There wasn't much Yellen could say that wouldn't have spooked the markets

The Fed chair's testimony to Congress in the wake of the controversial interest rate rise had to be circumspect

Interventions by central bank governors to soothe troubled financial markets come in two forms: the ones that have a modest, transitory effect and the much rarer ones that make a real difference.

Mario Draghi's "whatever it takes" speech in London in July 2012 fell into the second category. When the president of the European Central Bank said he was prepared to take on the speculators attacking Italy and Spain, the markets got the message: the ECB was prepared to use unlimited firepower to act as a lender of last resort. The speculators backed off immediately.