By then the main focus of US monetary policy had become the question of how to invigorate a flagging economy with short-term interest rates already at a historic low. In this context the purchase of longer-dated US treasuries was hoped to push down the yield curve, driving down interest rates also in those markets important for private investment and consumption decisions.
Operation Twist, the replacement of short-dated by longer-dated treasuries announced in September 2011 pursued an essentially similar tactic of decreasing the relative supply of longer-dated treasuries in the market, driving their price up and their yields down.