For the just the second time in its century-old history, the Federal Reserve has engaged in an attempt actively to twist the US yield curve to a flatter shape. This “operation twist” will have the most powerful effect if it marks a beginning and not an end to how far the Fed is willing to go down the route of unconventional monetary measures to rescue the recovery.

The immediate response from markets was disappointing. Yields had already priced in expectations of a twist. That by itself is no discouragement: it means that the move started working even before it was formally announced. But other markets paid less attention to what the new action may achieve than to the renewed pessimism by which the Federal Open Market Committee justified it. “Significant downside risks” cited by the FOMC, and borne out by bad economic survey results from China and the eurozone, have sent equities down and the dollar up.

It is premature, however, to judge whether the Fed’s shifting of $400bn of its balance sheet from shorter-term to longer-term assets will bear fruit. The answer will lie in market and economic behaviour over the next months, not days. But there are reasons to fear that little good will come from the twist.