That is the title of a new and very good John Cochrane blog post on fiscal stimulus. Here is one excerpt from Cochrane:
New-Keynesian models act entirely through the real interest rate. Higher government spending means more inflation. More inflation reduces real interest rates when the nominal rate is stuck at zero, or when the Fed chooses not to respond with higher nominal rates. A higher real interest rate depresses consumption and output today relative to the future, when they are expected to return to trend. Making the economy deliberately more inefficient also raises inflation, lowers the real rate and stimulates output today. (Bill and Rong’s introduction gives a better explanation, recommended.)
So, the key proposition of new-Keynesian multipliers is that they work by increasing expected inflation. Bill and Rong look at that mechanism: did the ARRA stimulus in 2009 increase inflation or expected inflation? Their answer: No.
The paper itself is here.