At Jackson Hole, Allan Meltzer offered a clear framework for the way such a policy direction might allow additional stimulus after conventional operations had lowered nominal short-term interest rates to zero.
In the typical model, money and bonds become perfect substitutes at some low interest rate, perhaps zero, and we have a liquidity trap where monetary policy loses its power to add further stimulus by lowering interest rates on bonds. Meltzer suggested this result reflects more the limits of the typical model than the limits on monetary policy.