Pushing on a string

Pushing on a string is a metaphor for influence that is more effective in moving things in one direction than another. If something is connected to you by a string, you can move it toward you by pulling on the string, but you can't move it away from you by pushing on the string. It is often used in the context of economic policy, specifically the view that "Monetary policy [is] asymmetric; it being easier to stop an expansion than to end a severe contraction."