Operation Twist: the name given to a Federal Reserve tactic to flatten the yield curve of the government debt by using the Open Market Operation to buy long term debt and sell short term debt. This monetary policy operation has been used twice inU.S. history. The first time was in 1961 and the second was in 2011. In September 2011, the Fed performed Operation Twist in an effort to lower long-term interest rates. In this case, the Fed sold short-term Treasury bonds and bought long-term Treasury bonds, which pressured the long-term bond yields downward.
Sources: Investopedia, wikinvest